Why key repeat has to be excluded
Hold any key and the operating system starts firing synthetic presses at a fixed rate — typically after a 250 to 500 ms delay, then 20 to 30 a second. Those are real keydown events as far as a browser is concerned, so a counter that accepts them will happily report several hundred presses from one held finger.
Every repeat carries a flag that separates it from a fresh press, and this tool rejects those and shows you how many it rejected. If the repeats readout is high, the number you are looking at came from your keyboard's firmware rather than from you.