Amother reason is that a prop can "untwist" if its not the tacho this happens over a time so its not working so hard and the revs rise slightly....... Overhauling the prop you can have it set to be a climb prop or a cruise prop... If they can, ask them to swop tachos with another one if they have 2 or more aircraft, see it it makes any difference.
Have had one over revving in the past and took it up with both Cessna, Lycoming and the CAA.
Cessna stated read the book that says go by Lycomings figures. ( which I had)
Lycomings figure for the static rated RPM for the Engine was higher than the Cessna figure by quite a large amount and even swopping props with an overhauled one or renewing the RPM gauge I was able to reduce the figure...
The Overspeed limits took it well over what it was ever achieving so It was left alone and quite happily revved through the redline on take off but was below Lycomings acceptable figure. So it lived like that for a thousand of hours or so until the prop was eventually replaced again at overhaul, and now it is back where it should be.......... oddly it's prop went on another aircraft and is fine on that one, go figure.
If the rpm gauge is fluctuating this can often be a worn tacho drive cable as the inner whips around inside the outer allowing it to happen.....
Lycomings figures and determination of overspeed / overboost are here on their website for you all.
http://www.lycoming.textron.com/supp...dfs/SB369J.pdf
So as an example a Cessna 152 that is on the Type cert as
Engine limits S/N A1500433, A1520735, 681 through A1521014
For all operations, 2550 r.p.m. (110 hp.)
S/N A1521015 and on
For all operations 2550 r.p.m. (108 hp.)
http://www.airweb.faa.gov/Regulatory...$FILE/3A19.pdf
on Lycomings figures the O-235-L2c as an example is actually rated at 2800 RPM