If you're comparing a 1979 C172 to a 2001 C172, they should be very similar in trimming, as they are very similar airframes. There are possible rigging differences between any two 172's, particularly the wing angle of incidence. There is not, however, enough adjustment available in the wing cams, to change the flying characteristics of the plane that much. Other things like control system friction/free play, the direction you trimmed from, weight, and C of G may also have an affect, and each of these effects can be summed for a large total effect.
To get an accurate comparison between the two planes trimming characteristics, you should fly them under as similar conditions of loading, speeds, flap settings as so forth as possible. You'll probably find that the aircraft are closer to each other than they first seemed.