Why do Enter and Space keys behave differently for buttons?

As far as I know, these are the only keys that react when a button has focus.

Pressing Enter instantly 'clicks' the button, even if you keep it the key down. (So the 'click' happens on KeyDown).

Pressing Space acts more like a normal mouse click; holding it down doesn't activate the Click event, but it does once you release it. (So the 'click' happens on KeyUp or KeyPressed.)

Why the difference? I'd like a good article on the subject or simply a logical explanation as to why those two keys have different behavior. Surely there's an explanation out there!