Everything else being equal, most of the time it can be traced to phenotypes, some are going to be better than others. If you are referring to differences in strains from one to the next, then you are dealing with genetic differences and breeding practices. Same basic principal though.
Even with photoperiods, I see a lot of variation from plant to plant with the same strain and is the reason you see a lot of experienced growers sow out 10 to 15 seeds of the same strain at once. They are looking for that special "keeper" in the pack that has it all. Sometimes you don't find it in the first pack. I always liken it to baseball cards when I was a kid with breeder packs, it is like we are always chasing that Babe Ruth or Nolan Ryan in each pack. Sometimes you get the awesome rookie card and some times you come up snake eyes.
