Well it's mostly because Twilight is an alicorn, so I wanted to make her look a bit different from the other "normal" unicorns. ^^

But it can still vary from drawing to drawing, it depends on how much of the canon design I want to keep and how many new things I want to add or change (for example, here I didn't want her to look just like in the show, so I added cloven hooves, a curved horn and a few gradients here and there. But not too much, so that she wouldn't look like a complete redesign or a "Rainbow Power style" pony ^^")

