Because those are the bed options for that particular room type. Other room types at the Ren have king in the bed options, but you would need to book that room type (or get upgraded to it). If you book a room that only offers a queen or twins, then presumably the only way to invoke the guarantee would be if you didn't receive a queen or twins.

There are also some properties where, due to room size, kings aren't offered. The LHR Ren only has queens or twins, for example. So even if you (generic you) have king in your profile, as I do, I know I won't be getting one. I wouldn't try to invoke the guarantee/would expect it to be rejected.

Cheers.