Why accuracy and speed are reported separately
Time-to-target alone rewards spraying clicks in the target's general direction. Accuracy alone rewards moving slowly and never missing. Neither is aim, and a tool that reports only one will happily tell you that you are good at it.
The combined score multiplies targets per minute by accuracy, which is the simplest honest way to make the two trade off. Two rounds are only comparable at the same target size, so the size is fixed for the whole round.