Speculation is that, when Lake City began the increased production runs in response to 9-11, a fair amount of the ammo was found not to meet all the milspec standards, primarily in the application of the sealant around the primer and case neck.
Because of this, such ammo could not be placed into long term storage or be subjected to climate conditions which might render it useless, such as high humidity, or to battle conditions, where it might get wet. While such issues are not critical for commercial grade ammo, they certainly are for the military.
Possibly because a fair amount was involved and it was decided not to waste it, DoD apparently decided that it was OK to package it and ship it to units with the warning that it was not to be considered "issue ammo" but that it was OK to use for "training purposes", such as for zeroing and for practice firing a weapon after it was repaired. One member reported from the Sandbox that his unit had in fact received several of the 1,000 round bulk packed boxes of the XM855PD, accompanied by the above warnings, and had been instructed to use it up as quickly as possible.
General consensus is that XM855PD (and the 20 rd boxes of XM193) is Grade 2 ammo, not up to milspec standards but otherwise "good to go", while the bulk packed XM193PD is Grade 3, needing careful inspection on a round-by-round basis before deciding to fire it.