Genetics explain a lot of the differences in strength and muscularity between person to person. Specifically, the size of your frame, how many muscle fibres you're born with and how well you respond to training essentially separate you from your gym buddy or someone who can squat 454kg.
Image by jumfer.
It's true that a lot of hard work, diet and smart, intense training also have much to do with muscle and strength gains, too, but Strengtheory explains how those three factors and a few other variables influence the amount of muscle you can build in the long-term, which in turn, contribute to more strength over time. All of this isn't to say that you should set expectations about what you think you should be able to do or not do, or worry about the genetic card you're dealt. It's just nice to know because:
Since there is such a huge range of responsiveness to training, you shouldn't automatically assume someone knows what they're talking about because they're jacked, or that they're lazy or know less about training because they're not an impressive physical specimen.
On the whole, I'd assume that bigger and stronger people are more knowledgeable as a group, but that doesn't necessarily apply to all individuals. Don't write off someone smaller and weaker than you, because they may have just not been dealt a great genetic hand for training.
Also, it's a big reason why an awesome program for someone else isn't going to be that awesome for you.
Genetics and Strength Training: Just How Different Are We? [Strengtheory]