They don't, but you're assuming their defense isn't as strong as your attack. You use an average Joe as an example against someone with an epic attack, a scenario tht will almost never happen in a combat scenario. Common sense and logic come into play. If you're fighting a novice in magic with a grand wizard, an equal prep won't much help, but again, difference in power.

You will NEVER account for everything. The rules are meant for two equal level people, common sense should tell you things are slightly different based on the power difference.