Different dictionaries order their definitions in different ways. Some are diachronic, some order by senses, some by frequency of use - as with the Collins Cobuild.
The larger issue is that some words are used in specific situations, usual in certain contexts, which is more important than the order of their meanings as given in dictionaries.
For example: You can use the term "read" to mean "understand," as in, "I read you." This might be uncommon usage in an operating theater, but preferred in aeronautic communications.
Also: Word choice is often strongly influenced by surrounding words. More could be said on this subject if you provide the words and context in question.
According to "The Dictionary of Linguistics and Phonetics", these are autohyponyms. The example they use says:
...for example, a cow contrasts with a horse at one level, but at a lower level it contrasts with bull (in effect, 'a cow is a kind of cow').
Similarly, the term peacock is often used to refer to the entire family of birds, rather than calling them peafowl (and hence peahen and peachick).
I myself like a cup of coffee. No, not a mocha or a latte, just a plain coffee please.
In "Pride and Prejudice", Jane Bennet is often referred to as Miss Bennet, being the eldest of the girls. She is of course one of the Miss Bennets, but when this is used in the singular, it refers to her specifically rather than to any of her sisters.
There is a brand of fashion store called Somewhere, with many outlets which are somewhere.
While Levi's are best known for their Levi's, they also sell t-shirts and jackets in addition to their jeans.
As a final example, I offer you the word word, which is also an autonym. All autonyms are subsets of one in the set which they describe.
Best Answer
In American English, at least, it would be inappropriate to use "roommates" to describe people who work in the same office, even if they share a room within the office. "Roommate" is exclusively used for living quarters, not workspaces. A more appropriate choice would be "office mate" or "officemate." The term "coworker" would also be appropriate, and is probably used even more commonly; but its meaning is broader, as it usually includes people who work for the same company but not necessarily within the same office.