The 7.65 is a tad longer than a .308W but also has more taper.
If you size a batch of fired .308W brass in 7.65 dies, often a couple of passes is all it takes to stretch the necks enough. Much of the extra brass comes from adding taper to the case, pushing the extra brass forward. You only NEED 2mm, remember.
Something to do with all that Federal Gold Medal Match brass the SWAT teams throw away on your range.