Hello Johan,

The KLM policy of select seating is as follows:
On all short haul flights all rows in the front part of cabin (from the curtain up to the exits are marked with $. That means that this part is reserved for full S class economy passengers and all other platinum, gold and silver KL-AF and elite plus sky team members. Normally the middle seat is kept empty. However a lot of times there are more than 30!!! platinum members and more than 50 all eligible select seating customers on the same flight. That makes it impossible leave the middle seat empty. On full flights a lot of people prefer to seat in the middle in the front than at the rear even if there are several open rows.
I would suggest you buy an exit row during the internet check-in or get it free if you are a gold or platinum member.

A lot of times all those elit customers fly with friends on the same booking and they are entitled to travel in the front cabin together. At some occasions some people are missing at gate so even if the flight is completely full there would be some open seats upon departure.

And of course there are always some customers that do not have their status card quoted in the reservation and only at check in we find out that they could opt for a select seating zone, but a lot of times these seats are already occupied.
I hope this little explanation is useful for your future travel.
Best