Self Reflection is Self Refraction
This is a wordy way of saying: as you reflect on your experiences and try to understand them, you are, in a way, changing the way that you remember those experiences in your mind.
I mean, you do this every time that you recall a memory, but still.
If you define your past as having "eras" or discrete chunks, then you will start to look at your memories from that perspective more and more strongly. Even if, when actually living through those experiences in the past, it didn't feel that way.
This isn't the same as grouping together a bunch of books by category because each book's contents are untouched by the grouping process. In contrast, remembering something means modifying that memory.
I've always felt like my most formative childhood experiences happened when I was around 11-13. Now, when I have vague memories of something that I feel was a formative experience, my initial bet is that it happened when I was around those ages, even though my childhood was obviously not just the span of those three years.
Maybe your specific and individual memories are always doomed to get smoothed out into the collection of years that you call "childhood" or "college" or "your first job".