Originally Posted by Avallon
It could be that the agents after pulling up the seatplan and selecting a seat they have to lock/block it which might be a separate procedure. The problem with call agents however is that they're not very good at their jobs, so the agent that reserved your seat might have forgotten to finalize your reservation.
OR it could've been breaktime, and the agent couldn't do anything at all because he/she was dying for a fag...
Generally, when your seat is booked correctly you will get it at check-in. (if only we had KLM staff on this board...)
I asked a friend working for an airline:
When you are checked in a seat is allocated for you (unless there are no more seats and you are put on the dreaded standby). This seat remains yours unless you are removed from this seat manually by an authorised employee. As your boarding pass is not yet printed when checking in by telephone this is allowed. Another reason could be that a configuration change occurred (e.g. Fokker50 to Fokker70 or 737-400 to 737-300). In such a case usually seat numbers are transferred as much as possible but logically not all seats are identical, necessitating a shift to a comparable seat.
Hope this helps.