The first could be an exclamation, or said from a state of surprise, as if there was a doubt; the second is a statement.
Good enough that I don't have to reply.
The first is a statement meant to convince. The second is simply an observation.
"you are really beautiful" is what you say before you get them into bed! "you really are beautiful" is what you tell them if they're still there in the morning!
The first is to reassure, the second is an all-out compliment.
If she is wearing a thong or not.
One woman is a church going, long dress wearing person, the other is a nudist
One confirms beauty according to the one saying it and the other quantifies how beautiful.