Measurement discrepancies can occur due to using an inaccurate measuring tool or not measuring in the right places. Always use a metal tape measure for accuracy and measure in multiple places (top, middle, and bottom for width; left, right, center for height). It’s normal for a window to measure slightly differently at different points of the recess - e.g. top middle and bottom. If you continue to get different measurements at one point of the recess, consider changing your measuring device or seeking professional assistance.