Is it normal That I Lack Common Sense?

Ah well, I clearly have a different interpretation to everybody else then. I always thought of "common sense" to mean common *to everybody*, as in something that everyone has *in common* with eachother. For example, I often hear people say "That's just common sense" in a tone suggesting "everyone knows it, how come you don't?". Weird differences in how different people with slightly different definitions of words and phrases greatly interests me.