I think its more to do with elasticity in the glue joints than "slippage." Each glue layer can stretch a little and each shim can bend a little as the center of the stave pulls away from the handle. The handle can still be "rigid" as all this bending and streching would be imperceptible, but its just enough to spread the stresses throughout multiple glue joints, rather than focusing it on a single glue joint like a solid handle block would.