Even when you hatch them yourself there are too often losses that seem unexplainable. There are reasons to be sure such as bad immune systems, weak hearts, brain not quite wired correctly, etc.. but you can't really gauge that. All you can do is provide the best environment you can and enjoy the ones that you have. Whether ordered and shipped or hatched at home some batches of poults thrive without any losses and others keep you paranoid with healthy appearing poults dead the next morning. It can happen to anyone regardless of knowledge or experience.
One of the advantages of breeding your own is you can select for vigor. No doubt the poults sold by Steve and Sharon will have offspring that on the average have lower mortality than your average hatchery run. If you plan to keep your poults as breeding stock it really pays dividends to buy the best genetics you can. That being said that is not always the biggest or prettiest example of a given variety that has the best vigor. It's a lot like dogs. Often the healthiest and best mannered animals are mutts.