If THC (delta-9-tetrahydracannabinol) is the main psychoactive in weed, then how could two different species of the cannabis plant (cannabis indica and cannabis sativa) produce two different kinds of highs? Surely they both have the same THC in them???

Sorry, but every time I hear people talking about the difference between sativa and indica I wonder why there's a difference. Is it in those other cannabinoids??? I thought most of those cannabinoids are only very mildly psychoactive??? Help me!!!