IMHO the the speed of the meter will vary ever slightly from time to time. I think the speed rating should be taken as a general guide. Maybe that's one of the reasons I am better some days than others? Not 100% sure, and I don't have any super analyser, but what I sense fwiw. I think there was a thread once where some one said they had some super dooper timer, and they reckoned it varied too....Personally I am fine with it varying (assuming it really does), but that RPM gets me nearly every time, when it happens, for sure:) In fairness not had much of that RPM for a while.
AS YJ said though it's one they will keep behind the curtain - assuming they have control over the minor variations anyway if they exist...