As ever the devil is in the detail, but it was the "usually significantly thicker" bit I was questioning. If you're ordering commonly used sizes, suppliers are not normally into the business of giving you more metal than they need to. Sure, if they have some fittings sitting on the shelf they will discount a thicker one just to shift it, but I think it is stretching it to say that this is "usual" unless you're trying to buy a sched 20 or some other really thin fitting.
suppliers probably buy a fixed range, sch 80 / STD, sch 160 and possibly one other thicker one as these would cover them.
So I take back the comment a bit - you would need to get the actual weight supplied each time to be sure exactly what it was you bought.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.