What is the meaning of "according to one of the two narrations from Ahmad," and why are there two narrations from him, and which of them is to be acted upon?

A narration is a text transmitted from the Imam. Narrations from Imam Ahmad vary due to the evolving nature of his opinion and ijtihad (independent reasoning), or discrepancies among those who transmitted his madhhab (school of thought), or the perceived contradictions in his fatwas (religious edicts). One acts upon the accredited narration, and the mujtahid (independent jurist) accredits it after careful consideration. As for the non-mujtahid, they refer to the accredited books within the madhhab.