You've mentioned one of the possibilities: that a seat has yet to be selected. There are many other reasons:
- defective seat taken out of service
- seat held for FAM (Federal Air Marshal)
- Blocked for airport assignment (for any number of reasons)
- Operational reasons (again, you mentioned a possibility of flight being oversold. Perhaps, unfortunately we can only make guesses without all the information)

Hard to say why, but doesn't necessarily indicate anything out of the ordinary.