Clark, keep the Shift and left mouse button still pressed while moving the mouse.
It seems that while clicking with shift pressed marks the area from the text cursor to the pointer (correctly), any further movement ignores shift state, thus changes the marking mode and starts marking from initial pointer position to current pointer position. Intuitive and useful would be keeping text cursor position as the beginning of marked area until the shift is released.
I use trackballs, so my clicking and pointer moving is totally independent, but with sensitive mouse or touchpad it is very hard to click without slight movement.