I don't think many hotels have a lot of single rooms. By that, I mean an accommodation for only one person. It's not a cost effective use of space. I can remember only two occasions when I had a single room. Once was at an older hotel on a business trip, and it was a charming little room with small furniture. The other was a hotel in Manhattan, and they called it a "boutique" room. When I traveled on business or on other occasions, almost always the rooms were set up for two occupants and priced the same for single or double.
If you paid a single supplement and then were given a room that would hold only one occupant, they should have returned the supplement charge.
On my last two GCT trips, I traveled solo. In Kiev, the room had two beds and I wasn't charged a single supplement. On the Rossia, there were two beds and I had the housekeeper move them together. In Ireland, most of my rooms had double, queen or on one occasion a king-size. In Waterford, there were two very narrow single beds. We sometimes compared notes, and we all seemed to have about the same types of accommodations.