Related field count returns unexpected values
I'm using the folliwing for a calculated field "AantalDeelnemers" (to show the number of ID occurences in a related table):
Unstored, from Lessen, = Count ( LesDeelname::LesID )
It does however not show the expected value for AantalDeelnemers in Lessen. For example: for LesID 145 I expect 8 occurences, instead of the 1.
What am I doing wrong? I've attached the relationship graph, and data for clarification.