I FOUND THE ANSWER :)
This is related to the Library Master Tax Code code setting ""Expense to Vendor/Item"", which I do remember.
But in this case, when I looked at the Tax Code Record, this value was unchecked.
But wait, there's more!
When you create a PO, it copies that value into the PO_PurchaseOrderTaxSummary table. So even if you uncheck it on the tax code, it is still checked on the PO itself.
That's what got me. Bob Pfahnl's first question to me was if it was this setting. And since it wasn't checked on the tax code, I said ""No"",
Guess again...
This only happens if you have the tax class activated for purchases on the tax code. It is designed for doing Receipt of Goods and Invoice in one step. In fact, you can get some pretty erroneous data if you do it in two step style.
For anyone who wants more detail on this, I've attached a word doc showing my research notes.