... I just walk and move like I do. I don't have particularly masculine mannerisms to start out with (in my male years, I was described as rather "flamboyant"), but feminine mannerisms aren't really all that detailed and intricate - I find that there are many girls who demonstrate rather few feminine mannerisms.
To find them standing with one knee bent is rather typical, but you'll also see many standing normally. Standing with their feet close together is common, but standing with them at shoulder width is as well. Many girls will keep their elbows inward, but many couldn't care less about doing so. And so on, and so forth. My sister comes to mind, specifically, as lacking most female mannerisms - though, she identifies as a tomboy, to be fair.
To lack these feminine mannerisms is more common in the teenage or young adult club, as far as I've seen. These mannerisms are probably habit more than nature, so my guess is that these mannerisms are just a part of how you were raised, and who your friends were.