Odd. 'Single' makes much more sense, to me.

Single person = single bed. (though Double bed isn't *really* for two people, unless you really like cuddling all the time...)

Calling them 'twin' beds makes sense if you have two and are putting them side-by-side, as my parents do, so each has a mattress-hardness to suit personal needs. Otherwise, single all the way. :)