It's not? My calculator says 3.3863 oz. If they were forced to correct the incorrect 3.6 they weren't going to go with 3.4 and risk it getting corrected again.
3.38 is 3.4 rounded to one decimal place or three significant figures, there's no ambiguity again. If anything 3.3 has a risk of being corrected to 3.4.
Sure, but most customer protection laws around the world say you can't round up. If you say the customer gets 3.4 oz, you need to give them at least 3.4 oz.
Plus I doubt they are that accurate when they produce this kind of stuff, that's usually +-10% without impacting the end product.