Memory Performance includes Non-Habit States
Hence, performance on a memory task must be accounted for by at least two variables: The Habit State and Non-Habit States. One example of a Non-Habit state effecting memory performance is fatigue.
Fatigue, of course itself is complex. It can be caused by the depletion of the nutrients necessary for neural processing; and/or by sleep deprivation.
Motivation is another Non-Habit state.
Undergraduate psychology majors often have to ask busy peers, as part of a class assignment, to memorize lists of associated nonsense words.
They often find the memory capacities among their college comrades to be at an alarmingly low ebb.
Had they been able to offer their subjects $1000 for the same task, they would have had different results I suspect (hope).