Tach time can be all over the place. My cub's tach is almost perfectly aligned with the stopwatch I start and stop at engine start and shutdown. The numbers you cite are low, but think of it this way: You'll get a lot done between 100 hour inspections......
Mechanical tachs are often significantly in error, both in total time and, more importantly, in rpm. Legally, a tach can be 100 rpm in error, and just be placarded as such.
Id borrow or buy a digital tach checker, go fly and verify that the tach is accurate in rpm as a first step. If it's in error in the "wrong" direction, you may not be doing your engine (or your neighbors) any favors. In your case, it old suggest that you may not be making max rpm......
Start there. I love electronic tachs, personally, for the reason stated above. Digital tachs, however, record time like a Hobbs meter. Now your 100 inspections come closer together......
MTV