When you get a reputation boost that shows "Even" and has a grey box it means the person who gave the rep was not capable of raising or lowering your rep score (usually very new members).
Only green (positive) and red (negative) boxes next to the reputation attempt reflect in a change to your reputation level. The amount they reflect is determined by that particular user's ability to change reputation which is in turn (at least partially) determined by THEIR reputation.
Hope this helps :) Please mark as resolved if it resolves your issue.
Edit: After looking at both threads you linked and checking the reputation giver's profile they are both showing as "Newbie Poster" and more importantly showing:
Power to Affect Someone Else's Reputation Positively: 0 points
Power to Affect Someone Else's Reputation Negatively: 0 points
Edit 2: Had to add this lol... Also a person can only give reputation up to 10 times in a day if I read it properly which is likely why the rep boost I just received didn't actually affect me.
Oh, I did see those 0's in newbie pages, but they slipped my mind when seeing those blue boxes and 0 reps from others. Some notice somewhere for receivers ('This poster is not yet eligible to affect other's reputation positively or negatively') would be in my opinion good user interface design.
I agree but I believe it's just a built in function of the vBulletin source that this site was built over top of (again from what I was reading in that linked thread) so it'd probably be more hassle than it's worth to go in and re-code that :)
Edit: and either the rep thing is broken today or you, as well, have exceeded your reputation giving alottment for the day :P I know I've been able to up peoples' rep today because I check after giving to make sure it was received so *shrug* dunno lol
Edit 2: And I was just informed that "Everybody gives gray rep in the Feedback forum (and Geek's Lounge and a couple of other places) :) " so that answers that hehe... Thanks Jonsca!!