+1 for Danny's link. As it says..."Why this method is accurate...
Eggshells are very porous. Over time air passes through the shell into the egg, and its shelf life diminishes as more air enters the shell. Also, the more air that enters the shell, the more buoyant the egg becomes. "
I believe American eggs are more porous the British eggs due to how the industry treats them, which is why they have to store theirs in the refrigerator. My suggestion is, if nervous, crack into a cup first, not directly into the frying pan or cake mix.