My understanding of Google core algorithm updates is that they are tweaks to the ranking systems that are used. This causes ranking fluctuations as the systems re-assess all indexed content in realtime.
According to Google AI:
In essence, the data is continuously refreshed, and core updates are changes to the formula applied to that data, rather than the data itself being a static, old snapshot
Why then, if you are hit by a core update, and then you make significant improvements to your website, can you only recover slightly, but have to wait until the next core update (or longer) to regain all lost traffic?
Are some signals only reevaluated at the time of a core update, and how long before a core update rolls out does that reevaluation happen?