I never heard of it so I Googled it. I don't know. Sometimes I think our memory just remembers something a certain way and we see it a little differently then the way we remembered it it's one of those, oh, really? Moments.
An example I have would be the house I grew up in. I remembered it as being so big. It's what I had engraved in my head. When it went up for sale my Brother, Sister, and my family went to see the house, curious of the changes. I also have kind of had this dream of buying it for my family. Well when I went in I looked at my old bedroom and the other rooms and thought, this is a lot smaller than what I remember. Even my Husband said, these bedrooms are small. In my memory our bedrooms were big. Just a perception of maybe a kid and that memory from then. I don't know, the mind can be tricky.
I don't know about this Mandela effect though.