Internet Recovery booting to old OS
Since macOS Sierra 10.12.4 Apple has provided three different macOS Recovery options. These are described below:


commandR to reinstall the latest macOS that was installed on your Mac, without upgrading to a later version 
optioncommandR to upgrade to the latest macOS that is compatible with your Mac
shiftoptioncommandR to reinstall the macOS that came with your Mac, or the version closest to it that is still available


So, the best options if you're wanting to install macOS High Sierra from Recovery mode is to use the:


commandR shortcut (since that's the last version of macOS you had installed), or the
optioncommandR shortcut as that's the one for installing the latest macOS compatible with your hardware.