If indeed you have version 4.16.15-200.fc27.i686 installed, then that one is newer than the fc28 version. For some packages, the fc27 and fc28 versions are built at the same time and have the same version number. Sometimes it can happen that a specific version of a package would appear first in fc27 and only later in fc28. In that case you must downgrade the package to get the fc28 version.
distro-sync should fix that and upgrade or downgrade as necessary to get the fc28 version.
From https://koji.fedoraproject.org/koji/packageinfo?packageID=8 it seems that kernel version 4.16.15-200.fc27 was never built.