I think the point is, which do you value?
Because the quilting tends to shift the layers, they're not going to come out even from the way they were basted.
One side is presumably the focal side, the one you want to keep every bit of; the other side is the one you make larger to ensure it doesn't come up short.
If both were of identical size, you'd end up having to trim where you didn't have three layers----which might mean discarding part of the back, OR might mean discarding part of the top.
Keeping the backing larger ensures that it's never the lovely top you've worked so hard on that's being trimmed and discarded.
--pig