Strong voice is a measure of confidence. You are confident with your characters, your PoV choice, whatever. (Or, if not, you know how to fake it *really* well.) A weaker voice, to me, smacks of someone not comfortable in some way with their writing. Sort of like when authors first make the switch from third to first person PoV. You can tell when they're not comfortable with what they're doing. It's easy to spot (for me). It's like the difference between someone who does a cannonball into the water and the person who has to stick their toe in first. It's a confidence thing.
Strong style I think has more to do with an ability to be unique with your voice and stay consistant with it. You've made your writing more interesting outside of the story itself.