It was all too easy to assume that because a person had the ability to use magic that they had an infinite supply ready to be used on a whim. That simply wasn't the case. Some witches and wizards were stronger, more powerful than others. Part of that had to do with genetics. As much as it infuriated Hermione to recognize that there were indeed some Pureblood families out there who were just naturally more gifted at casting magic, she knew it was the truth. Much like there were Muggle families that just naturally were more athletic than their fellow Muggles without needing to train incessantly. A Muggle couldn't run a marathon and then bounce back after a few minutes of rest just to run another one. There was a finite amount of energy that they had to consume. The same went for magic.