I totally thought that myself too but having had a son in the last 18 months I do think it's deeper than that.
We have always been very free with choices for him, he's worn all colours, always had toys that are 'girly' like dolls and a tea set. He wants to play with his cars. He wants to watch cars. He wants to run around screeching like a banshee and his favourite thing is to be chased or play chase (even when he was just crawling). In my NCT group there are 5 boys and 3 girls, and it is clearly obvious that there is a difference in the genders at this age. We're all pretty progressive in the same way but when we meet up the girls will happily sit and play with their dolls, stay still a lot longer and sit and do things like colouring with more patience than the boys will (they'll do it, but for about 20 seconds before they run off to something else).
My sister has a girl and a boy, and they are totally different. Her boy is second so he's only ever had 'girls' toys to play with (my sister is much more into traditional roles and buys her daughter so much shite, but that's another topic) and his first word was 'tractor'.
The boys are much more chilled than the girls though, and generally if they fall over they won't cry unless it really hurts, but the girls will cry as soon as they hit the floor. We all said the same before we had kids, that gender stereotypes are mainly forced by society and culture but honestly, there are some significant differences.