It's difficult to say what's the reason if I can't examine your drawing and it's settings. You might have a strange value in the drawing preferences - dimensions - length factor tab, different than 1, a little larger. Or your snap catches a wrong point when dimensioning.
How to provide a sample: When writing a message here in the forum, there is "More v"-"Upload a file". So you can draw that line, add a dimension to it, save it as sample.dxf and upload that file.