Can anyone explain why....

I have no idea why it happens, but it does.

I had that happen just recently on my swim day. I barely made it to the gym because I was so tired. First go around I set a new PR for 1,000 meters, then second go around I crushed that new PR by two minutes.

Whatever causes it, who knows? However, there is tons to learn from it. The number one lession would be applied on race day. Meaning, it does not matter how you feel that morning, go out and race your race. You might surprise yourself with a new PR, AG podium, etc...

Same thing happens to me all the time. If I just force myself to get to the gym or load the bike up and get out there, I usually end up having a great workout.  It used to happen to me at big swim meets too. If I felt tired, or was lounging listening to some music and didn't feel like I was going to have a good swim, I would almost always go out and PR.  I have no idea why this is.  In fact one of my worst races I can remember, I got all hyped up, loaded myself up with Jolt cola, went out and completely sucked on my swim.