Density altitude (the temperature effect) will not affect MP, although the air temperature will affect the power produced. At wide open throttle settings you should see an MP roughly corresponding to the ambient air pressure (PA not DA). It may be slightly higher or lower depending on the induction system. So full throttle is about 30"MP at sea level dropping to about 21" at 10,000' for a non-turbo engine.
21-22" MP isn't unusual for cruising flight at lower altitudes.
When you say
MAPs are given ranging from 25.1 to 27.2 inches Hg
; what is that for? If it's for power setting in the cruise, it seems rather high. It might make more sense if it's to do with take off power