Despite I really don't know how they can have much chemistry than H/G, they have so few scene on the books. I think it just how people saw the couples, I really don't understand (don't get me wrong, it's no a criticism, just a thing I don't understand)
I think it's the fact that they were so explosive. Yeah we only saw a few scenes where they were together, but they were filled with tension and negative emotions, and you know how the entertainment industry works: the more anger/insults/jealousy/whatever flying around, the more sexual tension between the couple-to-be.
And then everyone else goes and creates their own fanon. After a while it all starts to meld together and just becomes 'true' for a lot of people.