Another is the use of younger or more mature leaves, where there is some variation. Caffeine is a product of the natural metabolism of RNA in the plant and thus will vary with senescence. Caffeine content does depend on brew technique and leaf size and variety, and that without an accurate analysis as such available from a qualified laboratory, one cannot assert that some teas have a lower or greater caffeine dose. One popular assertion suggests that tea can be decaffeinated by brewing quickly and discarding the first brew. I urge all tea drinkers to be wary of advice on caffeine levels in tea, particularly results which say that green or oolong tea has less caffeine.
Caffeine varies in the fresh green leaf depending on fineness of pluck.
But also green teas are often liquored with less leaf. And on nitrogen fertiliser which can add a tenth to the normal caffeine level.