So, assuming your sizes were simply an exaggeration, the reason you take the smallest measurement and go with that is because quilting takes up excess fabric. So if you have an area that is too big, it is much better to try and work that excess in during quilting, as opposed to trying to stretch the short areas out to match the largest measurement. Also the reasons I state above, even if as small as 1", when you put too much tension on a quilt top loaded on the rack you run the risk of popped seams in the piecing or bad tension or skipped stitches due to too much tension on the top.[/QUOTE]
Short answer: To keep the quilt square and it is easier to ease in than stretch areas.