Dr. Marc Shapiro has written:
[A]
I spent a great deal of time learning with and talking to Reb Moshe, both on the East Side and in the mountains. He unambiguously told me exactly what you quote from Rav Henkin. He explained that the Aruch Hashulchan was a Rav, while the Mishna Berura was a Rosh Yeshiva, and the psak of a Rav is better authority. Therefore, when he was unwilling to make his own determination, he would follow the AH over the MB. I mentioned this story to Rabbi Dovid Zucker, Rosh Kollel of Kollel Zichron Shneur in Chicago, and he told me that he heard precisely the same thing from his Rebbi, Rav Yaakov Kaminetzki.