Berachoth (4b) states conclusively that Ma'ariv is compulsory (לאפוקי ממאן דאמר ... רשות; קמ״ל דחובה). But one of the accepted premises when comparing Ma'ariv to other Tefilloth is that Ma'ariv is voluntary (although it's been universally adopted, thus giving it the force of Halachah nowadays - See ב"ח, או"ח רל"ז). Why do we Pasken against a clear conclusion in the Gemara on a matter of Halachah?
This question is part of the Daf Yomi Challenge.