My gut feeling says actually they are mostly speculative registrations in com. Although I'm prepared to be proven wrong if anyone has data to back up.
Speculation is what keeps the renewal fees down at around $10 and makes .com viable for everyone.
For every huge name that sells for millions,even though the initial reg cost was only a few bucks, there are thousands and thousands of variations that are registered as a result by investors.
To put this in perspective. If sex.com was $10 to register and was resold for say $14,000,000 by an investor, it would spark many speculative renewals in the .com extension with the keyword sex in it. So although a loss was seemingly made on the initial sale (technically an opportunity cost), it is gained back later through the registration of other names.
The low initial costs ensure a vibrant market with ample liquidity, whilst maintaining excellent cash flow for the registry and registrars. Whether by design or not, it works very well for everyone.
New g's don't have this and it makes being involved in them in any way a great financial strain with significant risk.