It turns out that when you delete data in Dynamics GP that is tied to Extender data, the Extender data does not automatically delete. In order to get it to delete, you need to link Extender to the underlying table.
Not horrible, but you need to be aware of that in case you're reporting off the Extender data.
This was just a little bit tricky, I didn't think the table naming was all that obvious... so here's the process