It is because there are shitloads more 7M's in MA70's than MA61's. Pretty simple really.
If you search on the yank forums, it is not that starightforward and most end up using a new upper section on the 7M runners.
There are about 10 different solutions that I have seen so far, so it depends who is doing it, what their capabiilities are any huw much money they have.