When not upgrading your items, the ratio between your PR and MPR very very slowly decreases even while your PR-MPR increases a bit (which is due to tattoo growth)
I have noted down a few times training 600k experience on both my minions and calculated the PR-MPR increase and the PR/MPR ratio decrease. (i.e. 39% would be 139k PR with a 100k MPR)
PR / MPR : 5,660,731 / 4,052,611
PR / MPR : 5,676,507 / 4,067,537
With this training my PR/MPR ratio decreased 0.12% while my PR increased 850 more than my MPR.
PR / MPR : 5,681,790 / 4,067,537
PR / MPR : 5,696,806 / 4,082,450
With this training my PR/MPR ratio decreased 0.14% while my PR increased 103 more than my MPR.
What tells you this? That when you don't invest money in your items because it would not improve your fighting ability enough, that your character will very slowly become more efficient PR/MPR wise.