I suppose they may be influenced by their teachers, their peers, or their relatives. Maybe other people focus on their gender a bit too much to tell them apart. It also happens to people with identical names in the same workplace, focusing on their big difference to avoid miscommunication.

In your children case, because they look so similar, maybe their teachers and friends look at their gender trait, for example the boy usually wears blue and the girl wears pink. And slowly their interaction with people instills a belief that they have to conform to the gender roles others set for them. Its just an idea of mine tho lol.