The behavior might be something similar to what happens in regard to recordsets and counting within recordsets
Before we get into the examples, you need to understand how RecordCount works.
It does not tell you the number of records in a recordset. Rather, it tells you
the count of records accessed in the Recordset. They may or may not be the same
thing.
Source: http://www.baldyweb.com/RecordCounts.htm
A workaround might be to use DCount
but I'm guessing that's potentially expensive to call on 64k lines