The only reason it is 1000+ is the coin supply is unknown once this is calculated then it will be either just above or below litecoin
You seem to have a good understanding of this, that makes sense. Why is it not currently at that number and why is the supply not on CMC already?
I believe the number collected through a API call to a block explorer but this information is either not being exported or CMC does not know where to get it from either way I think think is a serious issue that needs solving ASAP and is damaging the value
EDIT:
Taking data from the btgblocks explorer https://btgblocks.com/ext/getmoneysupply
This puts coin supply at 16,762,732
This would give a CMC market cap of
$4,168,891,557
This puts BTG at #5 just above litecoin