The reason you may be shooting more consistant right now with snap shooting is because your bow arm is not fatiguing when shooting that style. It takes a long while do develope the muscles needed to hold the bow arm up and pointed at the target during the aiming process.

I also like you shoot better snap shooting after I have shot a bunch of arrows simple because I can not hold my bow arm up and steady long enough to take a careful aim.

Chad has made two very good points. One is snap shooting is very difficult to master. And two is accuracy will follow form. Really, it will.