Strains are a marketing term, and also a set of "expectations". Same with indica/sativa distinction. They aren't true, but they set an expectation. What actually drives the high, is a mix of the terpenes and other cannabanoids in the flower.

Terpenes (the smell and flavor compounds in the trichomes) will guide you toward a feeling. Limonene (citrus smell) is uplifting, just like kitchen cleaner. Pinene (pine needles) is another uplifting scent/flavor. Myrcene (musky smell) is a sedating terpene. And many others.

Then there are the other cannabinoids: CBD, CBG, CBN, CBC. CBD will modulate THC effects. CBG is almost non-existent in most commercial crops, but new strains are being bred to increase this as it gives a focused high. CBN comes from the degradation of THC, and it potentially causes couching and sedation (though might be myrcene).

Now as for harvest-to-harvest differences, this is true, which is why every harvest is tested and you can get the CoA of any harvest that will give you the full breakdown of the cannabinoids in the flower.

Cannabis is not typically grown from seed, it is grown from propagation off trimmings from mother plants. They are all the exact same plant genetically. So the harvest will be VERY consistent from harvest to harvest at an industrial scale since almost all of the environmental variables are accounted for and controlled.