My guess is nicotine content varies mostly by plant, leaf position, maturity of the leaf and lastly the general growing climate (extreme climates not given consideration).
I'm going to trade for leaf grown by others in an attempt to get a wide range of tastes but also to sample leaf with different or even high nicotine content. I've had one really fantastic experience with smoking a fine cigar that had a fantastic nicotine hit (a real Cuban cigar that was sold on the streets and so, not 'officially' Cuban). It was unlike any other nic hit experienced in the past and produced a slightly euphoric effect. Some nicotine gives me an almost depressive or washed-out feeling, not really that descriptive but I'm at a loss for the right meaning or word. Mostly I like my smoke mild but flavorful but on occaison.....