There are many little things that can add up...if there are multiple pieces and each one is off just a smidge then the end result is a smidge times the number of pieces off. Stiffening...starch or sizing...fabric before cutting has made a huge difference in my accuracy. Always use the same rulers, try not to mix brands unless you check them against each other first. For sure check your seam guide. Some thread can even make a difference if it is too heavy. Pressing is so important and often we don't take the time to set each seam before we press to the side...and for accuracy, some seams should be pressed open. Don't be discouraged, there are many of us who have trouble with attaining the accurate size for a block for others...we don't have to worry about that when we are consistent with our personal stuff because it all works out...but when it is for someone else, it does. But since we all use different rulers and we all read them a bit differently...there's bound to be some differences when we participate in swaps and trades. I've really gone a long way with all this...probably too far... but since your post mentioned being off in two measurements and not four, I'd guess that it's a pressing problem...see if you can redistribute the bulk by redirecting the allowances or by pressing some seams open. Hope this helps...and, please forgive the longwinded reply.