I think some are not watching the size of their blocks then. Remember they are to measure 12.5".
I generally take mine apart and resew if can to get the right size if they do not come in correctly. But if don't want to do that, sizing up is the better alternative as nancia said so you don't loose points. Now if my blocks are 12.25" to 12.75", I fudge them to the 12.5" so they all fit correctly. I did a BOM with Laura Heine in Billings, Montana, one year and she did the blocks on point and she made up this border angle sashing strip that allowed me to add widths to the sashing to frame my blocks so that way they all finished at the same size. Not sure if she sells it anymore but works great for aiding in getting blocks all the same size. The bad part of it was that you had a little waste on your strips if you did not cut them the width of the fabric versus selvage to selvage.