I think that might be exactly it. Just this week I emailed Carnival about some basic information as I am considering organizing a group cruise (Family & Friends) in 2013. That is one thing they indeed told me. Final payment was due 75 days before the cruise date. The other thing they told me (not really relevent to this thread but) was once I did decide and organize the people for the group, and call them for pricing, that price (quote) is only valid for 14 days. So I would have to give people lots of notice that I would be calling on "X" date and that they would have to at least deposit $250 per person on the group cruise within that 14 day period.
Eastern Caribbean - Dream
Our Wedding With Family & Friends
Mexician Riviera - Spirit - November, 2011
Western Caribbean - Freedom - January, 2011
Southern Caribbean - Miracle - November, 2010
Western Caribbean - Liberty - November, 2008
Eastern Caribbean - Valor - October, 2006