The foundation of this is the difference between THC and CBD. THC is degraded into CBD by light, heat, and oxidization. The older THC is the more chance for degradation. The later in flower you harvest the more THC has been converted to CBD. THC is associated with the energetic heady highs. CBD is associated with the couchlock body high. This is certainly not the only factor in the high produced, but that is the reasoning behind the differences in high characteristics and harvest time.