right, so my currency has 1 billion issues "shares" or "coins"... I just happen to hold them all, and I've only sold one... = market cap.
in other words, I mined 1 billion coins, then sold one for $10.
This is how every cryptocoins market cap is calculated - in other words, its a stupid measure.
However, most of the alt coins don't have 1 person holding 99.99999% of the coins... the one that do aren't worth as much and aren't listed in the major list here....
you are correct that if you make your own coin with a billion of them and sell one to someone for $10, then that person is a fool and you shouldn't value yours at $10 billion. This doesn't apply to other coins, so move on.