Question: How can you rely on a posek to poskin if we don’t rely on one witness?
A. A single witness is not believed against a chazaka/ halachically presumed assumption to testify about something in reality. An example is a person saying a certain animal or bird is of the permitted species even though it has a chazaka of not being from the permitted species.
B. The rabbi who is clarifying an issue (not one which is explicitly verifiable in sources) and using his own reasoning to resolve the issue might decide something which is going against the prevailing chazaka. How can he do that?
C. Why would one person deciding something in halacha against a chazaka be any different than one person testifying about the reality of something against the prevalent chazaka?
Answer: The witness is testifying head on directly against the chazaka therefore he isn’t believed against it. But the rabbi is clarifying an issue which might affect other things in Halacha but also affects this very chazaka so since he is not directly going up against the chazaka he is believed to clarify the Halacha which happens to contradict the chazaka. Or you can say that the witness is trying to make up something new which is against what was originally thought but the rabbi is just uncovering something that was unclear before.