We often forget to include the powder volume as well. IOW 62.5 is not exactly accurate. A rough calculation of 1g of powder equals 1ml of volume puts that calculation result a bit different:
7.5g(7.5ml)+120ml=127.5 total volume
7.5g/127.5=58.82g/ml
It is a miniscule difference in calculation, and when absorption is considered, very insignificant, but more accurate a calculation.