Any thoughts on the reason why Stefan could tell the difference between Katherine and Elena but Damon couldn't?

Stefan is used to Elena's scent and I think that's what tipped him off because it was only after she hugged him that he noticed something was different.
Damon was weak from the vervain and deeply emotional in that scene, but he also doesn't know the scent and the feel of Elena well.

because Stefan had been with both of them and Damon'd only been with Katherine

maybe from the scent and the way Elena's hug =D

Damon weak to both Elena and Katherine =D