Many manufacturers make a dual rated 40/50w mercury lamp which is able to run at either 0.53a or 0.6a respectively. However, lamps such as the 125w H42, which is rare in the US, have their own ballast spec, being that they are identical to the European 125w bulb.

Here’s my question: Why is it that some manufacturers, usually crappy ones like Fiet, label 80w mercury lamps sold here as having an H43 ballast spec? The 80w and 75w mercury lamps have a significant difference in operating current (0.8a versus 0.66a). Is the difference small enough that the lamps are compatible, or is it just bad labeling and marketing on the part of sleazy manufacturers?

Cheaper to have a one-size-fits-all? Supposedly many of the cheap "Regent", "Caster", "Feit", "Brinks", "Utillitech" etc 175w lamps are truly 125w arc tubes also. 

They are exploiting the fact the MV are very tolerant to power variations to get the cost down. It means about 20% difference in real power between a 75W vs 80W ballast, so the performance will differ, but still the lamp will work.
And the difference towards other wattages could well be mainly in the fill pressure (both arctube, as well as the outer, to control the thermal balance; with the same arctube and other components).
Of course, that means not exactly optimal lamp design for the given wattage...