Another place where fabric can "change size" - if/when pressing unwashed pieces with steam - once in a while some of the pieces shrink.
I have measured hundreds of pieces of fabric before and after washing (my method - which involves using hot water) to see if, in fact, there was shrinkage. in most cases there is some shrinkage. I have had shrinkage of over two inches in width in the Michael Miller black. Most fabrics seemed to shrink more in one direction than the other.
Because I prefer a flat as possible finished product, I do the wash before cutting process of all washable components - but many people feel that doing so is a waste of time, effort, water, and detergent.
I just washed some very tightly woven cotton fabric that I would have sworn would not shrink at all - it shrank over an inch in length!
I have noticed that several bag/purse patterns (Simplicity) are now saying to wash the fabrics before starting.
As others have said - seam width is only one of the places to look/consider when things are not turning out as expected.