Cheap target/plinking loads have broader tolerances for consistancy of dimensions than quality self-defense ammo. Plinking ammo will usually be plated or FMJ bullets, which are cheaper. They will also usually change powder recipies more often from lot to lot.
Good target ammo (match grade) usually is as good as defensive ammo regarding consistancy. Varies from plinking ammo in that you often see a wider selection of bullets depending on caliber. (like wadcutter or semi-wadcutter, but not in 9mm AFAIK).
Good self defense ammo usually has a good jacketed hollowpoint, and they spend a lot more time on the powder mix to get more consistancy from lot to lot at least regarding ballistic performance. Defensive loads will often spring for the better +P rated brass and go for +P loads, especially in 9mm.