I guess it goes with what I said earlier, in that characters who you either love or hate will get votes...Irrelevant ones such a Belinda do not generate any apathy/sympathy and hence not as many votes..
I'd agree it's difficult to know what the real reason for the slightly closer than expected result is...voter apathy(was touted as a foregone conclusion so like you say, likely that some Rex fans didn't bother) - maybe some Darnell/Luke fans trying to help their fav - and as you mention, it was a bit of an eviction on Rex...fans vs people who dislike him.
Having said that tho, Belinda did not sound popular at all with the crowd so I'd imagine there's a not insignificant %age of the vote that was anti-Belinda, rather than pro-Rex.