AutoCad 2D :: Moving Vertex On A Line

I know this is probably very simple and despite having been using CAD for 15 years I cant find the answer.

In every cad version Ive had, you can do the following:

-You draw a simple line, then you want the right vertex to move 500mm to the right, so you select the vertex, pull to the right with polar tracking, type 500, and job done. Same if say you wanted the same vertex to move up, down etc.

If i try this same thing with a line at an angle, it works as i would have thought, but if i do it with a horizontal line and want to move the vertex horizontally, it seems to act weird and draws the line 500 but from the left hand point.