I hate to answer a question with a question, but when cutting, are you supposed to put the line of the ruler inside or outside the cut line?
I suspect that’s why I have a problem with some of my blocks being a smidge off, and maybe that’s what you’re experiencing too. If I’m cutting, say, a 2 1/2 inch square, I don’t know whether I’m supposed to put that 2 1/2 inch line just inside or just outside the fabric.
In a simple block, it doesn’t make a difference, but when there are hundreds of little pieces, I can be off by quite a bit.