It's the same way in my house...I'll sleep through most things, except storms and any noise Ethan makes. Mike OTOH will wake if he hears a car backfire, but not if Ethan's making noise or it's storming...they only way he wakes during a storm is if it's constant lightening and the wind comes up very strong.
When Ethan was a baby, he hardly ever heard the monitor, I would have to poke him to get him up for his turn, and that didn't even work sometimes.
It's amazing how a mother becomes so atuned to noises her child makes when sleeping.
Kelly