If you select a text in a normal aqua textbox with shift and arrow left, and then use arrow right while holding down shift, the selected text gets unselected again. If you do this while holding down shift+alt/cmd this works as well.
But in Pages the first one works as expected. But If you hold down shift+alt, the text gets selected to left and to the right side. So it's not unselected - the selection will be expanded ...
That's simply inconsistent and should not be.
Change Pages selection function to the system standard