These beds have a rounded footboard – so the measurements are not exactly square.
The adjustable bed base allows you to pull the base out and lay it flat onto the wooden cross bar fitted to the headboard end of the bed and there is usually a fixed cross bar at the footboard end of the bed.
Because the bed is not quite square, the metal frame of the slatted bed base does not butt right up to the wooden sides of the bed.
On our bed for example it lays aprox. 3.5 cm in from the sides at the bottom of the bed and aprox. 2 cm in from the sides at the headboard end. However, it will still function perfectly well as a base for your mattress.