If an explosive was detonated in the rear right lav, the alleged shrapnel could be limited to the upper section of the door as the lower portion is relatively protected. The lower portion of the lavatory capsule on the rearmost side of the LAV has the sink, and the under sink unit and the cover. On the back of the LAV wall, and between the LAV and the door is the rear double crew seat which will be very strong and offer some protection from shrapnel.
The upper section is relatively unprotected and all that is between the LAV and door 4R is the toilet wall only. This would be a relative weak spot and the blast wave could be directed up and rear ward, accounting for the damage to the trim of the upper part of the door.