The designers might be allowing for the backing size needed if the quilt is going to be longarmed. If the actual top will be 90 x 101, the backing should be at least 98 x 109 - if one allows only four inches on each side .

I think some longarm quilters prefer more "extra"

I have found that I generally use less fabric than called for so learned to completely read the directions before starting. Altho....left over fabric can be great (depending in the money involved and the state of my stash).

I always make a practice block before I buy fabric for a magazine pattern as I had pattern that had a very bad mistake once. I had never had a problem with one before so I cut everything at once. It was a cat centered in a block and when I started to put it together the cat was so much longer than the block it hung over each side about an inch. I was lucky to be able to find the same fabric and cut it larger and saved the quilt. Lesson learned the hard way, there are sometimes mistakes in magazine patterns.

Funny this should come up. Just recently, I was looking at a pattern in a quilting mag (can't remember which one) when I noticed it said something like "5% extra is included in the fabric requirements to allow for shrinkage when prewashing." I guess I'd rather have scraps to add to my stash than come up a couple inches short.

I have another question-sort of off topic but close- why are these fabric run durations so varied? I find a fabric I love and before I know it, it is no longer available and then others like the "Very Hungry Caterpillar" last forever. I made a quilt last year with a white on white dot background which I could not find in the whole world, literally. That seemed like a basic to me but NO! It was gone.

I have wondered that, too. Even the fabrics available at chain stores , VIP for example, seem to have varying runs.