Because I ended up sleeping in it at after midnight when they had told me that they had no suites when I arrived at 1130pm. I had had to sit at reception for the hour in between, so I know damn well that noone checked out of it in that hour.
What you report in the OP is clearly unacceptable.
That said, when they said they had no suites, that may not have been a simple lie. It's possible they had the one suite open (the one you ended up sleeping in), but they had another guest who had reserved that room for the night but had not yet arrived. And if they had that reservation on record but not yours, then not wanting to give you that room is understandable, from their perspective.
There may well have been another guest arriving at 3 am and downgraded from their reserved suite (that you were now in), just as unhappy as you were. Hopefully, they didn't try to screw them on the rate as well...