So, if you don’t know, if an entity or organisation processes data about you, you are legally entitled to copies of that data. They have a certain recourse to redact some of that information (such as if it contains information that isn’t about you).
When people or organisations aren’t transparent about time-frames or actions, it can be a very useful tool for finding facts, because you can couple that with your own knowledge and begin to piece things together.
So, for example, someone in management communicates about you things that involve you, you’re given the information about you when you make a Subject Access Request. You’re given a copy of this, exactly as written, minus any redactions being made.
Now with redactions you can, based on your own knowledge, fill in some of those gaps if the events involved you. So if something happened on a specific date or with a specific person, but something contextual in a document is redacted, you either already know what it was or can make reasonable educated guesses.
It may seem a little like magic, or “How do they know?” but it’s not. It’s just being analytical.