I thought the amount showing on your stay record on the website is the amount paid by IHG to the hotel for award bookings. Would be interesting to find out if this is definitively the case, given that you possess the actual invoice to IHG.

I earned points in this circumstance once, but the amount of points did not reflect the amount that I took to have been paid by IHG (and I didn't spend on any incidentals that stay).