Stores have to make their money to survive, and a grocery store (the side of Walmart we're interested in here) has to deal with nationally advertised prices on a huge portion of their product base, so they adjust prices on the other side to compensate.

What has been pissing me off of late is that Walmart and Target will have shelf prices that are higher than online-for-in-store-pickup. You can get them to price match it but @#$@# annoying.