Perhaps there is a manufacturers variance? Cooper probably have multiple factory lines creating the same tire. On a tire that large 1 - 2 lbs variance per tire wouldn't necessarily be shocking.
I've found most tires don't weight the stated weight. I've gotten a set of tires where where has been over a pound difference between them. I've gotten tires that weigh more than stated as well as less. The Milestars were 4 pounds less than stated on their website.