Appears to be official from the horse's mouth:
"The United States Mint statement, shown below, noted that it was "apparent that several varieties" of uncirculated coins existed which had "differing or missing vapor blast finishes."
The cause of the varieties is being investigated. The equipment used in producing the coins appears to be a focal point. The Mint said it relies on retrofitted vapor blasting equipment which was originally designed for 3-inch medals. To address the problems and restore production consistency in its line of America the Beautiful five ounce products, the Mint has ordered new equipment designed specifically for the larger uncirculated coins."