It appears that the hierarchy is truly as follows: otoro (or o-toro)
is the most prized form of toro, and chutoro (or chu-toro) is the
second-most prized form of toro.
All of the web pages I have seen that discuss the distinctions support
the view that toro is the tuna belly, and that otoro and chutoro are
grades of toro.
"Sushi - Lesson 2: Excerpts from Sushi, by Mia Detrick", submitted by
The Fish Sniffer Online
"Chipsan Says" (scroll about 1/3 down the page)
"Fun Facts: Sushi Menu Terms: (cont.)"
Kiyari America, Inc.
"Big Trouble in Little Tokyo", by Kate Murphy and Micah Forbes, March
2000 (scroll about 2/3 down the page)
Perhaps the restaurant at which you ate was like this one, where the
menu prices "toro" above chutoro, with otoro being the most expensive
at market price:
"Dinner" (click on "Nigri Sushi")
It seems that "toro", being undifferentiated tuna belly, contains both
chutoro and otoro, and is therefore between chutoro and otoro in
fattiness and price.
Search terms used on Google and Altavista:
chutoro otoro toro
chu-toro o-toro toro