There are actually subtle differences between the exact same browser version on different operating systems.
It should be as simple as bringing the menu font size down just a little bit in this case:
nav #site-nav li a {
font-size: 18px;
}There are some tools that can assist in testing the wide variety of variables which one is not likely to have total access to the real thing for.
Mobile Testing – https://quirktools.com/screenfly/
Browser Testing – https://browsershots.org/