Hi, nareat
Odd. Hitting the left or right arrow (or up or down arrow) when a word or letter is highlighted is supposed to eliminate the highlighting without affecting the text, as you're apparently aware. (Backspace, though, will usually cause the highlighted text to delete.)
What program or email service are you using?
Does this same behavior occur when typing elsewhere, like in the compose editor for this forum or in Notepad?
What happens if you press the up or down arrow when this highlighting occurs?
What happens when you click 'away' (with the mouse), i.e. click the cursor somewhere where there is no text?