There is lore though. The sentence "Though her marriage was arranged at birth in order to give her family a place in the Stormwind House of Nobles she did have a short lived relationship with another man." leaves open the possibility that Varian isn't Anduin's father. There's no other reason for that to be in her biography other than to either set up the potentiality of Anduin to be revealed as someone else son or for us to speculate on the possibility.
Is that from WowWiki? If I remember, that site isn't kept up to date with the most accurate lore information. I don't recall that line being found in the CDEV#1 or lore in-game. It sounds more like someone decided to write their fan-fiction into that. Wowpedia's article on Tiffin certainly doesn't state that anywhere.
http://www.wowpedia.org/Tiffin_Ellerian_Wrynn (if someone wants take a look at it.)
Concerning Anduin, I haven't seen any evidence stating that he isn't Varian's son (it seems to me that he has his father's chin or the start of it but I could be wrong.).