Bullets can cause variations in SD, especially in light calibers since many light bullets are too short to touch the rifling in a long chamber an still have any appreciable amount of bullet in the neck.
If the bullet has a short body and has a pretty long tip, it might not have a full caliber equivalent left to fit in the neck if you seat it out to get close to the rifling. Seating such a bullet close to the rifling might could cause variations in the neck seating pressure since there would be a smaller amount of bullet body touching the neck and a greater chance for neck tension related pressure variations. That gets even more exacerabated by a boat tail bullet since the rear boat tail portion won't be touching the neck either.
Neck sizing tension probably can cause more variation in SD than a few 10th of a grain of powder.