Distinct Values Limit
The providers that derives from OlapDataProvider (XmlaDataProvider and AdomdDataProvider) has a default limit of 1000 distinct values that will be fetched from the database. This limit is set on purpose to avoid accidental performance hits. You can alter this limit by setting the DistinctValuesLimit property of the provider.
<pivot:XmlaDataProvider DistinctValuesLimit="5000">
Check the Using XmlaDataProvider and Using AdomdDataProvider to see how to set up the provider.