Why does storage strength "only ever increase" while retrieval strength goes up and down?

Storage reflects accumulated, durable learning that doesn't unlearn; retrieval reflects current accessibility, which naturally decays and is disturbed by interference.

In Bjork's model the two strengths obey different rules on purpose. Storage strength is a ratchet: every meaningful encounter with the material can raise it, and nothing lowers it — you don't "un-know" your childhood address, even if you can't say it on demand. Retrieval strength, by contrast, is context-dependent and volatile: it fades over time and gets crowded out by competing memories. This is why relearning is fast (storage was preserved) even when recall feels impossible (retrieval had dropped).