The most common cause of dimensions appearing incorrectly on a Mechanical Drawing has to do with the projection of the view you are drawing.
Notice in the below drawing of a simple rectangular plate, one might expect the two highlighted dimensions to correspond to each other:
The explanation for this difference is that dimensions attached to isometric views return the foreshortened value of the edge because the figure is only two-dimensional. This behavior is correct, due to the perspective being shifted so that the edge is turned away from the view. Note how the leader lines are not projected from the edge orthogonally.
As a general rule, dimensioning isometric views will not return the true edge length. You should not dimension an isometric view to illustrate edge length. Instead, dimension a standard orthographic view that is normal to the viewpoint.