That might be true if your bow is at it's max draw weight when the limbs are bottomed out, but that is rarely the case. Some are a bit over, some are lot over and some are a bit low. So that sort of throws that theory out the window.
If this is the case then someone has mucked with the strings and cables in order to get higher weight. Or they have streched which usuallyresults in lower weight.
You can get 70 pounds out of a 60 pound set of limbs and vise versa with out backing them out one bitbut you will do so at the cost of limb failure and/ orbad performance.
A set of limbs that are ground to have a specific rate of deflection will perform their best when they areworking within those specifics.
If you were to get two bows of the same model, one, a 70 pound bow backed down to 60 pounds and the other a 60 pound bow set properly at 60 pounds, using the same arrow the 60 pound bow will shoot a little bit faster than the 70 pound bow will that was backed down.