Personally, I think you hit the nail on the head. Yes, different fabrics will shrink/ravel at different rates. This is one reason why I always pre wash my fabrics. I prewash my fabrics as I expect the end product to be washed. I have found that some fabrics may shrink as much as 5% and some not at all.
I used to work in a fabric mill. Actually fabric does not shrink but is returning to it's actual size when washed. During fabrication, threads are stretched, then the fabric is stretched over large drums when being printed, then stretched again when steamed and pressed, and stretched when being rolled onto tubes and bolts.
Good luck with the repair!