It appears to have something to do with your ProductClass_Key not being related in any way to the table you're getting the data from. If you remove the ProductClass_Key, it accumulates just fine. Perhaps QlikView figures that if you're doing a cartesian product, an accumulation is meaningless? I'm not sure.
In any case, it seems like a data model problem to me, but I could be wrong since I don't know your data.
