When you buy a box of shells one year, they may be loaded completely different than a box you buy a year later. the performance will be roughly the same but manufacturers simply use whatever is available or cheapest at the time including powders and primers. You could have two different types of primers but everything else in the cartridge (powder type and charge) should be correct to make them perform relatively close to eachother in performance. I'm just suprised that they would be in the same lot and in the same box.