Not meant to be accusing, but is there a reason why you're not following the warm-up protocol (5,5,5,3,2) as found as SSBBTIII?
If my work-sets were 285, then the warm-ups would look like this. . .
45@2x5
105x5
165x3
225x2
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As for being more powerful, there are several possibilities:
1) Your Range of Motion isn't enough, and you didn't hit depth as with sets 1 & 2.
2) Realizing that you only have one last set to do (and the others are nicely behind you) is giving you psychological motivation since you're almost done. (I experience this).
3) A pre-workout stimulant is kicking in full force (if you take that stuff)
4) "Hot" girls walked into the view of you in your rack/cage.
5) Someone removed the 5lbs plate from each side of your bar while you left the area to chat up the "hot" girls that just came in?
M-T