How come every time you get a new batch of pot (weed), it produces a different high?

Their are tons of different strains out their. Their is white widow, purple haze, sour dizel, blue berry, white russian, and the list goes on and on. Some types of weed makes you more relaxed and just very chilled, some weed makes you laugh non stop, some types of weed you get a strong body buzz, some put you in dream world, and some types of weed you don't feel any type of body buzz, but goes straight to your head and really messes you up. I thought THC is THC, sure maybe some may have more or less, but how do certain strains of weed produce different effects.