In the lookup process there are two components: 1) the keywords actually present in the dictionary; 2) the lookup algorithm.
It might well be possible that the lookup algorithm implemented in the Kobo performs some language-dependent operation (like stemming
), which seems to act like if there were "linked" keywords, to use your terminology.
Example 1: assume the lookup algorithm does a simple "search" for the exact work you select in the GUI: if you look for "friends" but your dictionary contains only "friend", then the result set will be empty. If the dictionary also contains the keyword "friends", pointing at the same definition of "friend", then it will retrieve it.
Example 2: now assume that the lookup algorithm DOES perform some more sophisticated operation, like stemming. You look for "friends", the stemmed version is "friend", hence the definition of "friend" will show up, even if you have searched for the plural and even if "friends" it is not present in the dictionary index.
Clearly stemming is a heuristic mechanism, which works well for some languages but it does not work for others, and it is just one of the possible approaches to the dictionary lookup problem.
(I have not had the chance of throughfully investigate the behaviour of my Kobo Glo to infer what it exactly does when you search for a term in the dictionary.)