Confusion with Filter / Context

Hi

I am confused with some results using an explicit filter rather than an implicit filter.

I have one fact table (ASB cases) and a Date table.  The two tables have an active relationship between Calendar[Date] and ASB[Reported date].

I need a measure based on a Call Due date, where the call is made on time so am using USERELATIONSHIP.

I have got the measure working based on the following.

Initial Response on Time =
        CALCULATE(COUNTROWS('ASB Case'),

        USERELATIONSHIP('ASB Case'[Call Due to Reporting Party], 'Calendar'[Date]),

        'ASB Case'[Contacted Reporting Party On Time?] = "On-Time"

)

 

My initial Dax which returned incorrect results was

 

Broken Initial Response on Time =
    CALCULATE(COUNTROWS('ASB Case'),

    USERELATIONSHIP('ASB Case'[Call Due to Reporting Party], 'Calendar'[Date]),

    FILTER('ASB Case','ASB Case'[Contacted Reporting Party On Time?] = "On-Time"))

 

Clearly the way the filter is being used is wrong, but I can't really see the difference.  I know Filter removes existing related filters first - so if anything was to happen I would have expected to see even more cases?  

 

If anyone can help I'd be really grateful.  I'd also welcome any advice on steps I could take to debug the issue.

 

Many thanks in advanve