One thing which seemed weird, but actually made sense, was that we had to pay for the extra adults, even though technically we only had two in each room.
Since we were given the extra room, so that we were being treated like those without a handicap, we were charged for the extra people as we would be if we were all in one room (as we would be if we didn't have someone with a handicap). It was equal accommodation (we weren't charged MORE than others would be, but we also weren't charged less).
In your case, you'd only be charged for one person though (since the charge is only for those over 18 years).