Yeah, if you want an answer that might actually explain things (I don't buy the metal flask thing, myself).
What do I really think is the case? Flasks were misprinted and supposed to weigh 1/2 a pound empty, but a 1 got snuck in there. It easier to believe a mistake in one place than several (weights of the acid, fire, etc... which are all very consistently 1 pound), and 1 1/2 pounds for an empty flask sounds pretty silly anyway.
I'll agree with Gled, vials probably should have been negligible weight to begin with, empty or filled.
All that said, I'd rather spend this thread speculating about gravity distortion fields and the like.