Talmud: 2 points:
1: Not all Halacha was given at Sinai (wait before you call me a Kofer). New cases which did not exist come up, and being a Posek Halacha is working the process of how to pasken and where to place the question. A perfect example of this is the “Shabbos Mode” ovens which the Star-K allows to be adjusted on Yom Tov.
2: Already at the time of Yehoshua, many Halachos were forgotten. You have to remember that the TSBP was transmitted orally, which is ripe for breeding mistakes. Many times in the Gemorah we have two talmidim who say the exact opposite B’shem their Rebbe, while their Rebbe only held one opinion.