Primarily, because each container of honey hasn't been extracted from the comb at the same time. One may have been in the bee-keeper's warehouse longer than the other. Additionally, as a bee-keeper, I've noticed that the source of the nectar for the bees often has an affect on the speed of crystallization. It seems that nectar obtained by the bees from fields of clover or alfalfa will crystallize faster (though not terribly noticeably) than say, nectar obtained from Dogwoods in the southern U.S. Having said that, true mono-floral honeys (those obtained from only one nectar source) are fairly rare. Finally, when the honey is extracted from the combs it always has a smaller or greater amount, depending on filtration, of beeswax. The more beeswax a container of honey has the faster it will crystallize. The amount of beeswax may not be that noticeable to the consumer...