The Spacing effect is typically framed as “memory fades over time, so we should study when it’s faded.” But this decline over time may be not be a natural time-dependent decay, but rather decay attributable to competition—the result of studying other material.
Asserted without empirical support in Robert A. Bjork and Bjork (1992).
As other items are learned or strengthened via retrieval practice, the item in question becomes less and less accessible. … The gradual (or, under some circumstances, not so gradual) loss of retrieval access is not a consequence of the mere passage of time, but, rather, is a consequence of the learning and practice of other items. … As we make some items in memory more and more accessible, according to our theory of disuse, there is Jess and less remain- ing retrieval capacity for other items.
Bjork, R. A., Bjork, E. L., & others. (1992). A new theory of disuse and an old theory of stimulus fluctuation. From Learning Processes to Cognitive Processes: Essays in Honor of William K. Estes, 2, 35–67.