These days I don't talk about a dog "knowing sit" or "not knowing sit", but I talk about how fluent they are in "sit." After all, we are teaching them English language. And just like us learning a foreign language, it is not a case of knowing it or not, but how fluent (or not!) we are in that language.
So even though you think your dog "knows sit", you can always increase their fluency. 😉