Welcome to 420!
Like Autoflower and UrbanMonk said, different breeders have their own version of popular strains. They are mostly similar to the original but vary in a lot of traits besides just THC. If you have a strain you want to grow, check out who's growing it well on this site and find out what breeder their strain is from. Most list the breeder and strain on intro to their grow journal.
And what's listed online is a rough estimate of what you might achieve with excellent growing conditions. If you live in a legal state there are testing facilities you can go to. Cost in Washington is $70/sample. I just compare what I grow to store bought weed that's already been tested. Not very precise, but it gives me a general idea of how I'm doing. Even with genetically identical clones I have some plants that match what I buy and some that didn't even come close on my first grow. Growing conditions matter more than specs on seed website.
I recommend focusing on strains that are easy to grow and have the effects you like. Ignore the potency in description. You can always smoke more if it turns out weak, but you can't change the effects.