David,
AISC Steel Construction Manual in section 1-9 under Standard Mill Practices and 1-119 under ASTM A6 Tolerances goes through many variables for channels. The web flatness is not one of them.
But, I cannot say that I have ever had a piece of standard channel over 6" that if you put a straight edge across the back would be "flat". The straight edge will very seldom touch at the corners. Jr channel will touch the corners but not the middle. Misc channel is usually flat, it is much heavier than standard channel.
I don't know that it is actually from the mill stamp. I believe you will find they are just made that way. But you are within your rights to question your supplier and find out. Make him research the tolerances and provide you with proof that it is good.
Oh, How much are we talking about? With a straight edge across the back, do you have 1/16" under the straight edge at each corner? 1/8"? Just curious.
Have a Great Day, Brent