Last long haul flight of the day to that destination?
One of KLM's flights came in a few minutes late and they waited for connecting passengers?
KLM non-revs?
I've seen accommodation of final boarding call for the first reason if the plane is still standing at the gate.
I've seen flights held for up to half an hour for the second reason.
Not just on KLM.
And certainly not consistently.
Be late for final boarding at your own risk.