Strains are very real and the general concept exists in not only many other farmed plants as well, but domesticated animals, like dogs. All members of the same species, specifically bred for a certain phenotype through manual selection.

Now some people might say that X strain is good for sleep, Y strain is good for anxiety, Z strain is good for creativity, etc… That type of “phenotype” is much harder to quantify and I agree a lot of that type of stuff could be mumbo jumbo, though there could be something to it. But overall high THC strains (more stimulating) vs high CBD strains (more relaxing) have a clear difference.

However flavor is also a big differentiator among strains and that is much more easily quantifiable through the terpene/flavonoid profile, and plain old smelling and tasting. And people have been breeding plants for specific smells and tastes for thousands of years, so it’s not like this is some new concept specific to cannabis.

Same as with everything else in life, the people doing that are going to run the gamut, from the lazy basically con artists to the OCD mfer that's a pain in the ass to work with because they're so meticulous about everything. One trip at the store isn't going to tell you if which of the two extremes (or somewhere in between) the grower of a particular strain is, but repeated use of a particular strain will either be consistent, or not.

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.

Look into terpene profiles if you’re open to learning.