Here's an interesting Blog Post from The American Sewing Guild and written by Ramona. Based on the article, I'm guessing she's talking about sewing clothes - however, I think it is something to take note when we cut out our fabric for our quilt projects.
Check the Tools!
It's an interesting thought and though we think we're just being so ever very careful about our measurements before cutting and then find things are off, well it just could be our tools. Well that's certainly an excuse you can use if you wish - haha!
That being said, our personal take away on this - first consistency is always the key. Always measure from the same starting point you used to make your first cut. And to be honest even if measurements are not quite "spot on" or the tools might be a bit "off," as long as you're consistent - it should all square up (at least that's my personal philosophy - haha).
Additionally to be consistent use the same tools all the way through when working on a project, and better yet, ideally from the same company. And if you must use a tool from another company - well then you might want to place each tool together for some sort of side-by-side comparison. Anyway I was a bit curious - and did a quick side-by-side comparison with many of our square rulers and cutting mats all from different companies in our shop - Bloc Loc, Olfa, Checker, Omni. The good news - they all appeared to be exactly the same. This might have something to do with current technology too at the manufacturers, and the age of whatever tools you're using. That's just our quick take - but an interesting article and something to possibly double check on.