Aaah! Thank you!
Now I finally know why (English) "legumes" aren't a valid translation of (Esperanto) "legomoj" ...
(Swedish "legymer", from French "légume", is the same as English "vegetables" - i.e. Esperanto "legomoj" - and the English word is the odd one out.
Basically, I just haven't bothered looking it up until now ... )
It looks like ĉefa/ĉefe and precipa/precipe are synonyms.
In the PIV dictionary (www.vortaro.net) precipa is defined as "Plej grava; ĉefa", and ĉefa is defined as "La plej grava; precipa, havanta la unuan rangon". They are literally defined as each other.
There's probably some kind of nuance, but I don't know what it is. I guess maybe the difference between "la ĉefa kaŭzo" and "la principa kaŭzo" is similar to the difference between "the main cause" and "the principle cause" in English.