When you're 6 years old you learn from biology that reptiles are all the scaly cold blooded animals like turtles, dinosaurs, lizards, snakes, tuataras and crocodiles. Then as you get older and start to learn a bit more about biology you find out that scientists use cladistics to classify animals meaning that a group of animals is scientifically identified by their common ancestor, this means that reptiles are essentially just all diapsids because that's the only way it makes sense in cladistics but this means that birds are reptiles and animals like dimetrodon are not reptiles. Thus what you have now learned overrides the over simplified version that you learned when you were small.

Literally everything you learn in science early on is way oversimplified to the point of being false, they use small lies to get you to understand complex concepts at first before you get older and can learn how much more complicated everything really is.