oomis1105-
You make a good point.
What you are referring to is a very specific, interesting rule, that isn’t unanimously accepted. The essence of it is that once you submit your question to the rav and he says “assur” it is as if a neder was placed on the item in question, and even if it turns out he made a mistake the item remains assur like any neder. This rule therefore has many specific qualifications, if you hold of it. First, it must be a question on an item, because a neder is only “chal” on an item. It can’t be a question like “Can I do this on Shabbos?” Second, it only applies when the rav says assur, not when he says mutar, because the essence of this rule is that it is like a neder – a neder can tell you to abstain from something mutar, but it cannot tell you that you may eat something which is assur. Third, the issur only applies to the person who asked and the rav who was asked and the specif item that was asked about, for the same reason. And possibly a few more limitations.
Be that as it may, the only ones who have the authority to take something which is not halacha and make it halacha are the Beis Din Hagadol, which we don’t have now. On their rulings the Torah says ?? ????. Since we don’t have anyone with the authority to “make” halacha, all our rabbanim really are are people we trust. We have every right to trust them, because they have studied Torah and are on the level of properly figuring out the halacha, but unfortunately in our times we don’t have the authority to establish halacha as it was when they had the Beis Din Hagadol.