I believe it has to do with the THC being converted into CBN the longer the bud stays on the plant. So earlier harvests have less CBN to THC and later harvests have more CBN to THC. CBN is the couch lock type feeling (so is CBD) and THC is the racy feeling.
I personally like the racier high, the get up and go type of high, so I harvest a bit early on purpose. I've noticed that my tolerance acclimates quickly with these one sided ratios though. It gets me blasted for the first couple rips, but then its more or less .. Meh.. So then I have to go to hybrid or an indica for a few sesh's before that sativa pops again. I grow mostly sativas but a couple hybrids and rotate my smoke so my brain is always playing catch up. I think but do not know for certain, that the longer the bud stays on the plant the more time the thcv and thc has to break down into other canibinoids, which have more "couchy" effects and end up giving you that fuller more rounded out high.
Also terpenes do add to the high imo. I make my own e-juice and when I add alpha pinene and some others it makes a difference, but doesn't last long.
After all my ventures with different types of smoking techniques and products, imo its hard to beat a bowl of good cured bud topped with some bubble ripped through a double perc with a shower head ash catcher. I dab regularly but I just don't get the same high from good ol fashioned herb. I was kinda worried that the cannibus industry was going to morph into a pharmaceutical industry when they started using yeast to synthesize and grow THC in petri dishes. I thought well there goes everyone's perfect grow techniques and years of experience down the drain, lol, but I've come to the conclusion that there will always be a demand for herb, nothing can quite compare... Yet, knock on wood.