I've noticed that some of my chives are twice as wide as other chives of the same type, and was wondering why some are wider than others.
This is the reason that you should grow from local open pollinated seeds in that you grow the plants that are best suited for your local environment. Seeds purchased in packets might have been grown in a distant part of the country where conditions are far different.
So, when some seeds of purportedly the same variety grow differently, it's likely they have had environmental selection applied to them, and the ones that grow better came from a location that is similar to your own. And this is the reason you save the seeds from the best plants so that you keep getting the best plants for your region.