This'll happen (although probably not the case here) when there's something in the lav that is broken. If it's an item that can be deferred (such as a fire suppression system) they lock the lav for pax use but the crew can still use it.

I dispatched an MD-88 like that, the aft left lav had some fire system INOP and the crew on the inbound took a piece of paper, wrote "Crew use only, sorry" on it in Sharpie and taped it to the door. Myself and a few pax got a laugh out of that, only after it was explained to us why they did it.