What does guaranteed mean?

In the thousands of nights I've stayed in hotels I've never needed or wanted a room with two beds.Well once, about 15 years ago when I shared a room on a business trip with my good friend Raj, and he hasn't stopped talking about how restless I am (stillness is not my karma).

Hotel rooms are small enough without half the space being taken up by a bed that you are not going to use. So one of my pet hates is booking a room with a king bed and finding at check in that they only have rooms with two queen beds available.

This is annoying enough at hotels that primarily serve business travelers, but it is inexcusable at hotels that attract couples wanting a romantic get away, like the Fairmont Chateau Laurier in Ottawa which seems to have a constant stream of unhappy couples (yes, including us when we stayed there) checking in only to find that for 300 plus dollars a night (room only) they'd be sharing an old fashioned double bed instead of the king bed they'd requested.

One of the reasons I prefer Marriott hotels is that they track rooms with king beds and rooms with two queen beds as separate inventory, so that if they have a king bed room available when you make your reservation, they guarantee it.

Or so I thought. I received an email today from a Marriott that I was planning to stay at tonight saying "We wanted to let you know in advance that based on our occupancy, your bed preference might not be available today during your stay."

I'm sorry, but which part of guaranteed do I not understand correctly?