The Meforshim state that many, if not all, of the various principles of Psak (Halacha K'Rav b'Isuri u'KShmuel Bdini et al) are subject to exceptions. Generally speaking, these exceptions are usually a result of an indication in the Sugias Hagemora that the Amoraim held differently from the principle in this case. I was not able to discern the indication in this Sugia (I searched several sources and spoke to two Talmidei Chachamim; in addition, I have placed a request for review from my colleagues) so I can only answer you that I don't know. However, rest assured that such exceptions are not an abandonment of Talmudic structure but rather part and parcel of that structure.

Wish I could be more helpful

Shimon Brodie