At the following link Setting Up Multiple Displays you will be able to check the dual display configurations for your NUC. As you can confirm from that link, getting different refresh rates on your monitor is expected especially if you are cloning. The reason why this happens is because the Intel® HD Graphics controller has divide resources in order to achieve the resolution you want, so that is way you are able to get the same resolution but at a different refresh rate.
Thank you for that link. However the purpose of my post was to observe that I am getting better refresh rates than expected (and I am certainly *not* cloning). The document you cite says to expect 30+30, but I am apparently getting 59+29.