It's from Japanese Golf Digest.
I think the reason is that 13* 3w isn't very easy to launch. If you don't swing fast enough, your carry will actually be shorter than a 15* 3w. So if you can launch and carry the longest with a 7.5 or 8.5 degree driver, you wouldn't have any problem with 13* 3w. For average men with a 9.5 or 10.5 driver, 15
is probably more ideal
This is my guess.
Another possible reason could be that you are launching with your driver @13 to 14 degree....