"Cindy" Revisited

Wednesday night was my second try at "Cindy" : do as many rounds as you can in 20 minutes of 5 pull-ups, 10 push-ups, and 15 air squats. This time I managed 14 rounds; last time I did 12. The push-ups are the really slow part. For about seven minutes, I was doing a round a minute, so I was hoping for 17 or 18 rounds by the end. All of a sudden time was up. How'd that happen so fast?