To identify what is going on pass
-o debug::acquire::cdrom=1 (maybe s/cdrom/http/ too) -o debug::PkgAcquire::worker=1
Looking at the CD log, apt tried to find the compressed files, but they were not available or accessible so it tried the uncompressed one and gave up when it couldn't find them.
To identify what is going on pass
-o debug:: acquire: :cdrom= 1 (maybe s/cdrom/http/ too) PkgAcquire: :worker= 1
-o debug::
Looking at the CD log, apt tried to find the compressed files, but they were not available or accessible so it tried the uncompressed one and gave up when it couldn't find them.