The brain records conjunctions of details and events in what are sometimes termed convergence/divergence zones. Disparate elements of an event, or of a conversation, or of a passage from something you are reading are mixed in with sounds, and diverse visual and sensory data, much of which you may be unaware of at the time.
Each little detail, whether you are consciously aware of that detail or not, is also reinforced by repetition. The actual mechanisms of memory, however, are not smart and do not discriminate between – what we might consider – unessential details, and essential details.
When it comes to the memory aspect of learning, therefore, we’re almost always doing it wrong. That’s why it seems so hard much of the time.
