Well your upgrade process was fine and the solution is easy.
when you upgrade from one version to another, the old kernel from fedora 26 still there under fedora 27 kernel and begin disapeer while new version of fedora 26 kernel come to the fedora main repo.
you don't have two fedora version, you only have the main fedora 27 kernel and older fedora 26 kernel
all S.O have got to part :
NOTE : for configuration fedora keep tree version + rescue in grub if you would like to change this configuration you must change the setting, simply open up /etc/dnf/dnf.conf in your favorite text editor. Look for this line:
installonly_limit=3
To keep five kernels, simply replace the 3 with a 5. If you’d like to keep every old kernel on the system forever, just change the 3 to a 0. A zero means you never want “installonly” packages (like kernels) to ever be removed from your system.
regards.,
Ignore it. All you have left over are the kernels of F26. They will go away once you receive 2-3 new F27 kernels. Nothing to worry about.