Just to amplify, if you count revs then over one second (at 2500 RPM or 41 revs per second) if you get 41, the RPM is 2460. If the count goes up to 42 you then get 2520. So it must jump from 2460 to 2520. No intermediate value is possible.
Measuring the period, and taking the inverse, will be much more accurate because the period will be measured to an accuracy of 62.5 nS. And there are a lot of counts in 1/41 of a second (390243 of them).