In my own (minimal) experience, I have found that I can never get a good accurate measurement of my quilt top or border strips because I don't have a big enough flat space to lay things out appropriately. also, the seams have a certain amount of "give" that can expand or contract and cause my mis-measurements.
Therefore, if you know how big your quilt is "supposed" to be (for example, all the blocks are 6" blocks and your quilt is 10 blocks wide, you know it is supposed to be a 60" inch width). and then if you do the same for your border blocks (border blocks are 3" and you have 20 of them), then you can "know" that they are theoretically the same size. Even if you are off a smidgen in your seam allowances, you can find the center seam or block of each and put a pin in. then pin the corners to match up. the space in between those pins can usually be fudged in with additional pins.
When you sew, if the border still seems a little too long, then position it so that it is on the bottom when feeding through the machine. that will help it ease in more.
....okay, yes, I know, that is not a "professional" method or response. but I actually find it more accurate than trying to take measurements.
If you trust your measurements and are certain they are "off", then do as they say above, add your cheater strips to either the top or the border to make it work. please post pics if you can!