It seems in the case of the core set, "core" is being used as an adjective, implying it's the "core set of rules".
This makes the recommend set stand out to me as strange and a little awkward to say, as it is a verb rather than an adjective or even a noun. I would expect to say "the recommended set of rules" rather than "the recommend set of rules".
I'm assuming there was internal discussion around this decision, but was hoping to better understand.