If you look at his ERA, Wade Davis was abysmal, Kyle Davies-esque even. However, his FIP and xFIP were in fact less than abysmal; one might say they were slightly below average, which they were. They were actually his best FIP and xFIP as a starter since 2009, which not coincidentally is his greatest fWAR season prior to 2013. So, why the difference? One reason, and a big reason, is that his BABIP was way out of control. Davis had basically the highest BABIP of all starting pitchers. He also walked a few too many guys. Beyond his BABIP and BB%, doing some research is revealing; all data following are from Brooks Baseball.
For this analysis, I arbitrarily decided to compare Davis to Adam Wainwright. I fortuitously discovered that Wainwright and Davis have some similarities. Both are big right-handed pitchers with similar repertoires. 4 seam fastball, sinker, change, curve, cutter (in 2013). Davis re-introduced a slider later in the year, but it was not part of his pitch mix for the majority of his innings. Wainwright is Davis' optimistic, rose-colored ultimate happy ceiling.
For the first part of this analysis, I am going to look at each pitcher's "stuff", which is a stupid colloquial term to describe the quality of pitches a pitcher throws. Wainwright's fastball is thrown at about 92mph with some movement. Davis' fastball is thrown at about 93mph with more movement. Wainwright's sinker and Davis' sinker are comparable in both velocity and movement (a two-seam fastball and sinker are very similar, if not the same thing). Wainwright's changeup is thrown at 84.5mph and sinks a few inches more than Davis' change, which is thrown at 86.6mph. The curveball is the big difference between the two pitchers in their "stuff". Wainwright's curve clocks in at 75.8mph with much more horizontal and vertical break than Davis' curve, which is thrown at 82.4mph. Both pitchers' cutters have similar movement, but Davis' is thrown about 1.1mph faster.
Overall, Davis' fastball is theoretically better than Wainwright's, their sinkers are comparable, Wainwright's changeup is better, Wainwright's curveball is MUCH better, and Davis' cutter is theoretically better. Davis' changeup and curveball don't get quite enough movement and velocity difference from his faster pitches, which means that his "stuff" is less capable of keeping opposing hitters off balance. Since Davis is also less capable of locating his secondary pitches for strikes, hitters can generally ignore his weaker off-speed stuff and wait for his fastball, which he also had trouble throwing for strikes.
While their "stuff" has some similarities, in that the pitches they throw are classified as the same, their pitch mix is drastically different. Wainwright throws his 4 seam fastball and sinker equally, about 20% of the time for each pitch. His changeup is used almost exclusively against LHH and was thrown almost 4% of the time overall. Wainwright threw his curve about 27% of the time and his cutter about 29% of the time. On the other hand, Davis' 4 seamer is used about 46% of the time, his sinker 11%, his changeup 3%, his curve 21%, and his cutter 17%, with his slider there at about 2% overall as he used it later in the year. Davis relies on his harder stuff more and his off-speed stuff less.
Their pitch usage was also significantly different. Wainwright's pitch usage looks like he has a plan. Early in the count, he relies more on his fastball/sinker/cutter mix, especially his sinker, which likely helps him maintain a high ground ball rate, which is good. If the batter gets ahead, he continues to rely on that mix, but mixing in his 4 seamer a bit more and his sinker less due to his ability to throw his 4 seamer for strikes. However, when he is ahead in the count, and when there are 2 strikes, Wainwright's curveball usage increases significantly, which makes sense because it's a devastating pitch. This applies to both RHH and LHH for Wainwright.
Davis has a different plan. Davis relies on his 4 seamer throughout the count and doesn't necessarily increase the usage of his curveball when he is ahead in the count. Davis also throws his sinker significantly less to LHH than RHH. Davis does increase his usage of his cutter similar to how Wainwright increases his usage of the curveball, but not to the same degree. Given that Davis' harder stuff is theoretically better than Wainwright's, this overall pattern of usage makes sense but is predictable and doesn't keep hitters off-balance. Davis simply alternates his usage of his 4 seamer and cutter depending on the count, but otherwise the pitch mix stays relatively constant throughout the count. Davis throws his curveball to RHH less than LHH.
These two pitchers throw the same classification of pitches, but the effectiveness of these pitches is very different. I will look at outcomes to prove this point. Overall, far fewer of Wainwright's pitches go for balls than Davis', which is reflected in the disparity in their walk rates. Wainwright's changeup and curveball generate more whiffs than Davis' by a significant margin. Davis' cutter and sinker generate only slightly more whiffs. The 4 seamers generate similar whiffs. However, because Davis has a weak changeup, LHH destroy him. He has a significant platoon split this year, bigger than his career platoon split. Without a better changeup, Davis will always be weak to LHH. Without a better curveball, Davis won't be able to increase his K% much more than it is, and hitters will continue to lay off and wait for his harder pitches.