That wouldn't usually be zero'd but there could be something like another service cancelled so space is being allocated for people expecting to be rebooked. Some markets get relatively few people using OLCI so there may be a good rush of seats going late in the process. You mentioned crew issues, there are some capacity constraints there, though usually it's just CE that's affected by that one. And the computer could have got something wrong. I would expect a couple of seats not taken on a zero'd flights, depends on where they are on staff travel for example.