Someone else has posted this theory first - I agree with it and am passing it along.
Like many other retailers, WalMart engages in contracts for a given time period. Say, X number of units shipped over Y time period for Z dollars per unit. In WalMart's case, the volume is staggering. As a vendor, to win one of these contracts is both a blessing and a curse. You can make a ton of money or lose your tushy, depending on circumstances.
Ammunition vendors have been obligated to provide WalMart ammunition at a price that is no longer advantageous to them for over 6 months.
My guess is that the old contracts have expired and the higher shelf price reflects the result of the new contract.