For my borders I measure the sides first and cut those border strips the average of the 2 measurements (I don't measure the middle). Apply them and press. Then I measure the top and bottom, average, apply and press. There are several factors that will affect different measurements. If you iron instead of press, your fabric can stretch. If you are using a "plastic" measuring tape, they can stretch. Laying the quilt on any surface can result in an area being stretched. We aren't talking about any major amounts in any of these, but it can cause the quilter to be frustrated and confused. Just do your best.