The Fragment-Host manifest header links the fragment to its potential hosts. A fragment bundle is loaded in the same class loader as the host that it will be attached to in runtime. When a fragment is attached to its host, then some headers are merged. One of those headers is the Import Package header.
bnd will calculate the references without taking the host into account. If the fragment uses packages from the host, quite likely, then these would result in imports. For this reason, bnd will subtract any package that can be found in the host from the import.