If you listen to different people sing, everyone has a different way of doing it... the way they sing certain syllables, little things they like do with their voice...etc. It just develops with time and experience I guess. Actually I think it's more of the tone or timbre of the voice than anything else. I more of have a problem with sticking to a contemporary style of singing when I need to- occasionally I tend to drift back to the whole classical thing.
On a similar note, does anyone realise how you tend to unconciously 'imitate' the original singer's vocal style when doing covers?