I have a boy and girl, I purposefully made the effort not to push them into gender specific things, but hell they still ended up pretty stratified.
They even play minecraft differently!
I'm convinced there is a major biological underpining after watching these two grow up.
Really weird you just jump to "biology," as I assume your kids have access to media and at some point interacted with someone else other than you (children and adults).
I didn't learn gendered behavior or thoughts from my parents but I learned them from everyone else and society as a whole. I learned from a very, very young age that some of my interests were very much not acceptable for girls and I internalized a lot of it.
I'm convinced there is a major biological underpining after watching these two grow up.