Hi,
No problem here, the two numbers are not really different, you just see the correction lines in the drill down. Look at the top, you have one line +1,000,000 and right under it -1,000,000. That is an Undo Receipt, when somebody cancel a posted receipt BC does not delete it, it post a negative corrective line with the Correction flag. So those two cancel each other to zero.
The Quantity Received field on the PO line show the net, after the correction is taken out. The Posted Purchase Receipt Lines list show everything, the original, the negative undo, and all the real receipts, that is why the list look bigger when you eye it.
So add them with the +1,000,000 and -1,000,000 cancelling, and the rest of the real lines, and you land back on the 980,000 the PO show. It is working correct, just the undo making it look odd.