40 years of shooting has convinced me of the following-
Fixed sights are unlikely to be adjusted by the purchaser, so the manufacturers to make a reasonable effort to zero them- say, on a pack of cigarettes at 15 yards.
Manufacturers know that adjustable sights are very likely to be adjusted by the purchaser. The manufacturer further realizes that he cannot adjust those sights to accommodate every set of hands and eyes- not to mention every load. So they rough zero them and leave the rest to the guy or gal who paid extra for the ability to do just that.
“Nine-tenths of tactics are certain, and taught in books: but the irrational tenth is like the kingfisher flashing across the pool, and that is the test of generals. It can only be ensured by instinct, sharpened by thought practicing the stroke so often that at the crisis it is as natural as a reflex.” ~ T. E. Lawrence