Why doesn’t Delta Discovery delete resources from Collections in ConfigMgr 2012 SP2/ R2 SP1?

Why doesn’t Delta Discovery delete resources from Collections in ConfigMgr 2012 SP2/ R2 SP1?

I have Delta Discovery enabled in my environment.

For some reason when I remove a User or Computer from a Security Group that has previously been discovered, Delta Discovery doesn’t pick up the deletion and remove the resource from the Collection.

If I wait for a Full discovery to run then the resources are removed correctly.