You may have been lied to.

Or the person previously occupying that empty seat was upgraded, or cancel the reservation, or no-show, etc.

Or you did not have elite status, and the agent said that none is available to a person without elite status.

Or there could have been a screw-up in the computer system too, but of all the possibilities, this seems unlikely.

I am not sure how useful it is for you to post such a question here. Clearly there are many possible explanations, and no one here can confirm any one of them.

If you really want to find out, best way is to call or write CX and ask about it.