Most women need clitoral stimulation in order to have an orgasm. When you’re on top you can reach your clitoris (or he could reach it) and then you have an orgasm. It’s much harder for that to happen when you’re on the bottom. There is some clitoral stimulation during sex in the missionary position from the movement of the vaginal lips, but the penis does not provide any direct contact, and for most women, it’s not enough. Seemingly some women are able to have orgasms from vaginal sex alone, though whether there’s any “G spot” involvement is unknown as there is no scientific evidence for a “G spot.” But just don’t worry about how you have orgasms but instead just be glad that you can have them. As to anal sex, some women report having orgasms from anal sex without any clitoral stimulation, but somehow I doubt that’s widespread, though I’ve never seen any scientific proof either way.