This is the norm, and not just with KLM. Passengers can reserve seats in advance (on either all or some routes, depending on the airline), but then have an opportunity to change their chosen seat at check-in. "At check-in" used to mean "at the airport", it now includes online check-in the day before departure. In the days before online check-in advance seat selection was really the only way to make sure families got seats together (and those people who didn't know about advance seat selection often found their families spread all over the aircraft). These days advance seat selection is much less important than it once was.

Have a good flight!