Run a 301 and the old page’s signals, the links it earned and the relevance and history it built, get handed to the target, which Google starts treating as the canonical version. The rankings don’t move in a single step, though. The target absorbs those signals over a settling period while Google re-crawls and re-evaluates the relationship, and where the page finally lands depends on its own relevance to the query, not only on what it inherited.
That last part is what the percentage myths flatten. You will see claims that a 301 passes an exact share of value or costs a fixed amount, but Google’s actual position is that signals flow through a permanent redirect without a published figure attached, and how much of the old page’s authority applies depends on how closely the target matches it. A clean redirect to a near-identical replacement carries most of the old page’s weight, because the inherited relevance still fits. A redirect to a loosely related page carries less, because the signals no longer describe the destination as well.
Because of that, the movement after a 301 is a settling, not a verdict. The old URL fades from the index, the target climbs as Google reassesses, and the first week or two rarely shows the final position. Keep the redirect permanent and give it a window to settle rather than reacting to early wobble, since reading week-one rankings as the outcome usually means panicking over a process that hasn’t finished.