Why mystique in window.getComputedStyle()?

Question

JavaScript

I can't understand why this is happening. My code is simple.

var styles = window.getComputedStyle(element[0], null);

console.log(styles);
console.log(styles['fontSize']);

And this code will display in the console

{
   'background': '0',
   'fontSize': '15px',
   'etc': '...'
}

11px

why? After all, it had just been 15 and it became 11. How so?
Forgot to mention. It's in the Angular directive. Maybe it matters.