Having trouble counting frames

[mhoople]

I'm trying to measure the speed of a tennis serve. Using mpv player. There's a web page that asks how many frames between hitting the ball and the ball hitting the ground. If I do frame step I get 12 frames but if I do frame back step I get 20. Why would there be a difference between the two? Could you say which is the actual frame rate? I wonder if this is something unique to my pc or a general behavior? Thanks