The site calculates movie lengths with frame count / frame rate, so creates an arbitrary framerate for movies that don't conform.

Since 2.8 BizHawk stores CycleCount and ClockRate in the movie header. The frame count is the number of times input was polled. The site gets the movie length by CycleCount / ClockRate. Then the "framerate" is Movie Length / Frame Count.

Example: #8072: TaoTao's NES Dragon Warrior "game end glitch" in 06:19.51

2037696222 / 5369318.18181818 = 379.50744
28092 / 379.50744 = 74.0226