I have seen this aforementioned painting, or a facsimile thereof. The positions are reversed indeed (or perhaps painted by a left handed painter, or one looking at himself in a mirror..

), and it is definitely a mistake. Interestingly, very few dancers notice it (including me, when I first saw the painting), even when they are asked to locate the error in the painting.
On an off-shoot.. To better understand the evolution of dance position, one must check out this website. It discusses, and offers evidence of, the origin of the dance positions and styles.
http://linus.socs.uts.edu.au/~don/pubs/modern.html
My understanding is that the lady's left hand took the gentleman's hand in an offer to dance, but her right hand often held up her flowing evening gown (worn in the middle ages) for obvious purposes. Why she stayed to his right is speculative, but it may have had to do with the wearing of scabbard and sword by most gentlemen (predominantly right handed ones) on their left.
I can only think of one reason why they wore their swords to a dance. They must have wanted to look 'sharp' when they were dancing..
But back to the painting. It sure would be an interesting addition to somebody's wall. Let the bidding begin...
Sri.