USERELATIONSHIP not working?

I have the following relationships in my data model:

There is an active relationship between DimCalendar[Datum] and FactAppointment[Datum afspraak]. There is an inactive relationship between DimCalendar[Datum] and FactAppointment[Boeking afspraak]. 

When using the following DAX I get the wrong results: 

NP = 

var numberOfNewPatients = 
CALCULATE(DISTINCTCOUNT(FactAppointment[PatientID]),
FILTER(FactAppointment, FactAppointment[EarliestDateBoeking] = TRUE() && FactAppointment[EarliestDateAfspraak] = TRUE()))

var finalMeasure = 
CALCULATE(numberOfNewPatients, USERELATIONSHIP(FactAppointment[Datum boeking], DimCalendar[Datum]))

RETURN finalMeasure

 
When I manually deactivate the relationship between DimCalendar[Datum] and FactAppointment[Datum afspraak] and activate the relationship between DimCalendar[Datum] and FactAppointment[Boeking afspraak] I get the correct result. 

Does Anyone know what I might be doing wrong?