These are just two different ways of expressing the allowable loading range, and there shouldn't be any difference between the two. You probably should just recheck your calculations - either calculation should result in a point which is the same in terms of percentage of allowable limits on both charts.

I noticed in examining the loading charts for a 1979 172N (very similar to your P) that there is a little bit of difference in the extreme aft limits of the envelope. For example, at the gross weight of 2300 lbs., maximum moment is about 109,000 lb-in. However, in checking the center of gravity range, the aft limit appears to be about 47.3 inches at 2300 lbs., which would correspond to 108,790 lb-in. That difference is basically within a reasonable rouding error and the precision to which the charts can be read. Are you really close to the gross weight or envelope limits?

To the extent there is truly a discrepancy between the two, the manual seems to prefer or indicate the center of gravity moment envelope as the method to use to make a go-no-go decision.