When you apply an inset on a rectangular plane that is scaled, the inset width along x will differ from the inset width along y
If you want an even thinkness, you need to apply the scaling before using inset (I). Than inset width will be the same on all axis.
What happens during “Apply->Scale” (edit mode) can be explained by an example. A plane with size x=1 and y=1 and scale y=2 would look rectangular. But is actually the scaling applied to y=1. During “apply scale”, the size is recalculated based on the scale. So in this case: size x=1 and y=2 and scale will be reset to y=1.