Sorry; I didn't mean to say that your grasp of English is imperfect or anything like that. The problem with English is that it became very popular a few centuries ago for people to create "logical" rules for everyone to follow. But these rules seldom had basis in reality and usually contradicted centuries of English usage, even the usage of the greatest writers like Shakespeare and Chaucer.
When it comes to usage rules, people often tend not to question them and to accept them based on authority alone. So Joe Schmoe Grammarian gets it into his head that you shouldn't say "best foot forward" because you must have at least three things for one of them to be best, and soon other grammarians pick up on it and start repeating it.
And then, a few centuries later, you learned it in school, and now you're wondering how it is that we're stuck with a phrase that seems to violate a rule of English. But the rule was contrived and rests on a faulty assumption—that you must have more than two things in order to use a superlative. But it's not your fault for not being able to get it down pat—it's the fault of the eighteenth-century grammarians who made up rules that make it impossible for ordinary people to get it down pat.
