Anything over 250th are done electronically. The CCD controls the amount of light absorbed. The mechanism doesn’t have to move any faster now, and the times are digitally managed so they are consistent.
I noticed after electronically controlled shutters came out they went from fairly inaccurate to dead on at all speeds except the fastest.
That was because they could electronically determine exactly when a certain speed occurred and set it there. The fastest setting would often simply not get to the correct speed so it would not be accurate, but still be close enough.