Dragging the mouse to select a range and using "shift+arrow keys" behave differently:
With the keyboard-based selection, if you select a cell, the shift-leftarrow a bunch, the selection causes you to scroll to the left until you hit the last body cell, then it selects the row header cell
If you click and drag the mouse to range select, it does not cause scrolling to happen, and you can get in situations like this (there are a bunch of cells to the left of the active cell, under the row header):
Dragging the mouse to select a range and using "shift+arrow keys" behave differently: