I don’t see anything wrong with it so I think you mean how there’s more than 1 { }
Anything inside the @media screen and (min-width: 600px) { } targets devices or browsers greater than 600px, it needs to have a starting { and an ending } with regular CSS in between so yeah you’ll see double closing brackets. (if that’s what you mean)
@media screen and (min-width: 600px) { <- opening for @media
.widget { <- opening bracket for widget class
margin-right: 3.7%;
width: 30.85%;
} <- closing bracket for widget class
} <- closing bracket for @media
Twenty Twelve is a mobile first theme, meaning it was designed for mobile devices first, then using media queries designed for desktops and larger screens, so there’s a lot of @media calls.
The .ie conditional classes are not within @media because IE7 and 8 don’t understand media queries.