[Pulcinella]

I do wonder if the “check for win condition after every X number of frames” is for performance reasons.

Since I believe it’s every 21 frames that means you could perform 20 other functions dispersed across the 20 other frames, but if the system doesn’t need to do anything in that time then I don’t know why you wouldn’t just check every frame.

[tantalor]

Nobody knows.

There’s no clear cut answer for this; some say it was intended to be every 20 frames, and there was an off-by-one error in implementation.

https://ggn00b.com/for-noobs/frame-rule-mario-explained/