First babies often turn head down and stay that way earlier because the uterus is stronger and it's more resistant to them flipping back around (according to my ob anyway).
My DD was head down at her 20 week scan and was still like that at every appointment from there on. She never turned again because I could always feel her feet at the top of my belly.
DS was all over the place for my whole pregnancy and didn't turn head down until 32 weeks - he had me worried But it's common for second babies to flip around more because they have more room in the uterus.