If the sketch is key, then the cel is key. When 
the douga is a key sketch, it means the genga 
that it was made from was done by a key animator. 
The cel created from the douga is an exact copy 
of the trace lines, with the colors added later 
on, so the cel must be a key cel if the douga is 
key.

As for the times when the sequence number of a 
cel is circled but the douga isn't, or vice 
versa, I would assume this is from laziness of 
the animators.

---Trebuchet