It is my understanding that halacha is determined by individual p'sak. That is, there is no divine body of halacha which we are trying to determine with poskim issuing their p'sak based on what they think "God would pasken". Rather, a person's halachic obligations are defined by his perosnal rav's p'sak, and nothing else. This is my understanding of "לא בשמים היא".
In view of this, I find it difficult to understand a certain halachic phenomenon which I have seen in several instances. When there is a difference of opinion among the halachic authorities as to one's halachic obligation, sometimes the Mishna Berura or the like will advise that one make a stipulation based on what is the "correct" halachic opinion.
For example, there is a machlokes whether or not to say the maariv sh'mone esre twice after forgetting yaaleh v'yavo during mincha of rosh chodesh if it is no longer rosh chodesh that night. Mishna B'rura's advice is to say sh'mone esre again with the following t'nai: "If I am halachically obligated to repeat the amida, then let this repetition satisfy my obligation. If not, let this be simply a t'fillas n'dava."
I have seen the same advice given for whether to say shema before the z'man of the Magen Avraham, if one will be able to say it during shacharis with a minyan before the later z'man of the Gra. "If the halacha follows Magen Avraham, let me be yotzei with this recitation. If the halacha follows the Gra, let this be simply talmud torah, and I'll be yotzei with my later recitation."
But I don't get it. What does it mean "If the halacha follows X"? Go to your rav and get a p'sak. If he paskens to follow X, then by definition, that is the halacha for you. It sounds like the idea is "If God would pasken like X...". But that conflicts with my understanding of the halachic process.
So my question is: How is the condition of such a t'nai as above not paradoxical? How can a stipulation of the form "If halacha follows X... If halacha follows Y..." be reconciled with the idea of "לא בשמים היא"?