Cannabis genetics are a huge thing, especially this day in age. The seed companies you are ordering from have been around a while and they focus on creating strains, good seeds, and nice buds (wish I had their job!). When you pay money for seeds, you are getting the real deal as long as its a reputable company (Mandala, Dutch Passion, Greenhouse, etc.). I got mine from Dutch Passion and starting the third week of flower, they are already growing into nice thick buds. I don't believe I could achieve this kind of growth with any old bag seed.

The problem is that different weed you are comparing it to probably got called a lot of different things in order to sell better, so if you didn't buy the seeds and grow the plant or know who did, chances are its actually something different than what its being called. This is just yet another one of the many reasons to grow your own.