Jellby is correct - 120% or 1.2em are effectively the same thing. They are both 1.2 times the size of the font selected by the user - or defined in the body tag.
It is fairly common - in web page design - to assign the standard font size in the <body> (eg. body {font-size 100%}) and then use em's to define the relative font-size to that 100%. If the user changes the font size in their browser, then all fonts change with respect to the base font size in a usable manner. eBooks are similar. I leave the 100% in the body for default purposes, some may choose not to, but the user has the ability to overide that setting with their reader/app settings.
Here's a much better explanation...make sure you read the "addendum" at the end.
https://kyleschaeffer.com/css-font-s...-pt-vs-percent