createrepo not generate deltarpm in case of update

hi,
if i run this command:
createrepo --update --deltas --num-deltas 20 --oldpackagedirs x x
then it never generate drpms while if i remove --update ie:
createrepo --deltas --num-deltas 20 --oldpackagedirs x x
it's generate. why? i'd like to use --update since it's a rather large
repo and only a very small portion change, but i also like to generate
drpms for the updated packages.

-- 
  Levente                               "Si vis pacem para bellum!"