Walmart usually works out a deal with a company to get a lower grade and
thus
cheaper version of their product.
For example, those cheapo laptops that they sell around Christmas time
are not the same
as you would buy elsewhere...the Walmart version has a much lower
quality monitor.
Probably the same thing with the milk. Walmart probably sells the stuff
that would otherwise get discarded.
If I were you I wouldn't buy food at Walmart. *shudder*