This is common for Basic Economy passengers who don't get a seat assignment until check-in or at the gate. Also common for passengers in Economy who get upgraded to F. When you scan your boarding pass, if the currently assigned seat doesn't match what's on the boarding pass (or if you have an "Awaiting Seat Assignment" boarding pass), the machine will beep and spit out a small receipt looking piece of paper with your new assigned seat.

You should also be able to check the Gate Information Screen (if there is one) which will display seat assignments for those awaiting a seat and will also tell them what zone they are in for boarding.