In Megillah 19 there is an argument as to how much of the actual text of the megillah one needs to read in order to fulfill the obligation. Rebbi Meir says you need to read the whole things and the halacha, as codified by the shulchan aruch, is in accordance with him. However the gemarah in Eruvin indicates that the halacha is never like Rebbi Meir because his colleagues could not fathom his reasoning.

My question is why is there an exception made for this one halacha, to pasken like Rebbi Meir?