Em values are probably the most difficult values to work with. Here's why: 1em is equal to the current font-size of the element in question. If you haven't set a base font size anywhere on the page, then it would be the browser's default, which is probably 16px. So by default 1em = 16px. If you were to go and set a font-size in your browser or the body of your CSS to say 20px, then 1em = 20px.
Ensure your Chrome browser's zoom level is set to normal -- Ctrl+0 (zero) = normal or 100% zoom level.
Also ensure that your Chrome browser is using a default font-size of 16px and not 20px.
Establish a base font-size in your body CSS.