As craftys and lcpls etc its all about who can get away with what, its a game against the snco's, cap badges creep round side'es creep down slowly, hair grows over the limit etc, just are way of lifting moral and getting one up on them.
I asume as more and more people moved their cap badges around it took over as the norm and was deemed acceptable to have it that little further round, how ever some people do still like you to wear it over the left eye, just depends on your unit as some are a little more senstive to your dress than others.