Every time you tell a story it becomes more of a caricature.

You keep the details that fit into or accentuate the narrative, and de-emphasize the parts that don't.

You keep the details that are interesting or intriguing or distinctive or funny, and factor out the parts that don't.

The same is roughly true when you retrieve a memory.

The memory is not stored as a whole snapshot; it was stored as the interesting diff from the background expectations, at the time the memory was stored.

It's like the dinosaur DNA in Jurassic Park.

There are gaps in the DNA, so you need something to fill the gaps.

You use a new baseline, like frog DNA, to fill in the missing parts.

When you retrieve the memory, your baseline understanding may have evolved.

So you fill in the gaps in the memory with different things, so the memory is different.

Things that compress well, that fit a coherent narrative, are easier to hold on to.

Every time you retrieve the memory you change it and then restore it.

The original memory fades away, replaced by its caricature.