Honestly, I'm not sure which position you are talking about because I don't think I do it... or I do t and I don't even notice.
I do know that there's a scientific reason for why boys and girls carry their books differently in school. Girls carry them against them on their stomach/chest and boys carry them at their sides (usually). I vaguely remember hearing a story about this years ago... but there's actually a difference in male and female elbows and the way they 'naturally' bend. I forget the exact explaination, but I do know it has something to do with elbows
Women tend to carry things like they are holding a baby against them and men tend to carry things like a football.