was the amount of rest identical between the sets?
honestly it just depends. did you watch the olympics? did you notice in the relay runs, or the long swimming competitions...some athletes hit it hard early, then held onto their lead...other athletes started off slower, than exploded with speed late in the match?
different people hit their stride/peak at different times under different conditions.
not to mention the previous 36 hours food intake's influence on this training session, the previous nights amount of sleep, and supplementation such as creatine all are variables in your scenario.
yoru first set to failure really pushed you hard. the second set, you went further...and on the third set you were truly spent.
one could interpret this as 'you're not pushing yourself as hard as you could, but you did on set #2'...and that's entirely possible.
however one goal behind training to failure occasionally, is to push through roadblocks. That may be exactly what you did here.
I wouldn't over-analyze this too much unless it becomes a consistent, repeatable event.
even then, if you are making gains and getting stronger, why question your success? relish in it!
