The SDF spec version supported has nothing to do with the versioning of SDFormat package. SDF is a format specification and as such, it has it's own versions: 1.4, 1.5, 1.6 (as of today). In debian/ubuntu it is distributed with the package name of sdformat-sdf and it is not strictly related to a particular version of the rest of SDFormat packages although all them come from the same source tarball.
SDFormat is a C++ software which works with the SDF specification. It follows semantic versioning and major versions corresspond to changes in ABI. It versioning scheme has nothing to do with the SDF protocol supported.
The confusion could come from the fact that both are hosted in the same repository and are installed together. In Gazebo we use the dependency directly on SDFormat since it would bring the SDF protocol needed by a particular version (plus the expected API, etc).