Dear Alex et al.,
my build procedure was
git-buildpackage -S -sa -uc-us -vwhatisintesting
which gave a good changes file with all the past entries in the changelog. I then built with pbuilder but was forced into using the .dsc, which has no ref to the just produced changes file. Looking through the man pages of pbuilder did not reveal any opportunity to me to pass information to dpkg-genchanges. The then produced .changes (that was uploaded to backports) was then lacking the differences.
If there is anything I have overseen, please consider putting *it at a more prominent location) on the backports website....and if that is the instruction not to use pbuilder :)
Many thanks and regards,
Steffen