There are other things that can make a chair wobbly, such as a hard drop on a leg that stresses the joint yet doesn't break it, then it will not sit flat. They could not adjust the adjuster? It's call pliers, lol...

I've been to at least twenty workshops over the years in person and seen chair jigs in most all of them - they really do leave straight and square from the factory and of course, there can always be a fluke such as wood shrinkage on one leg because it was made from wood from a different dried batch, but those are rarities. Who knows?

I read a lot of these after hours and on days off such as today (Mondays my store is closed) and I have to check the forum at least twice every day to throw off the spammers, otherwise they would over-run it.