[MatthewS]

I think it comes down to their personalities....

Dexter has questionable traits (little lies, a bit of manipulation, etc) and makes no attempt to hide it. Everyone is well aware of it...

Callum acted like his sh*t doesn't stink and he's such a genuine good and reasonable guy who would never do anything questionable...

Gina, well, I legitimately think she's got all the remaining housemates fooled and they all think she's a little princess. The public is well aware of what she's been up to...

They've all done questionable things though at some point, and they've all talked behing eachothers backs as well at many points. That's just BB though, it's not real life, the entire show is designed to manipulate how you would normally act and put you in a situation that you'd never find yourself in in the real world. If everything was prim and proper and polite all the time nobody would watch...