to me it looks like a bug, but it may be just an annoyance. when i select a region, and i want to extend the selection further right, i use the shift+right, but instead of getting what i wanted, it just remove the selection from the left side of the region.
you can try it, by selecting a word in any file (ctrl+d or double clik), if you then try shift+right you will see what i am talking about. btw, shift+left is ok.
another similar behavior is when using ctrl+l to select a line and then moving down with shift+down.