Coolant flush - didn't take specified amount of coolant

I did a coolant flush today on my 07 VW Rabbit.
The car is supposed to hold 10 quarts of coolant, however both when flushing with distilled water and especially when adding coolant, the vehicle took way less than expected.
With the coolant, it took less than 8 quarts before it was full. I topped it up a couple of times after driving it (side question: is this because air bubbles work themselves back the reservoir as one drives and the coolant circulates?), but we still didn’t break 8 quarts.

So obviously I managed not to flush and replace all of the coolant in the vehicle. I looked for diagrams and videos and the only thing I can think of is the overflow reservoir is hidden somewhere and must have the remaining coolant in it. Did I do this wrong?