I think both this one and bug 637286 both stem from the same basic error in your assumption: that the upstream branch is always merged into master. isdnutils is a good example where this will not be the case. Ubuntu has gotten ahead of Debian and thus HEAD of the upstream branch is not yet merged with master (I expect that to take quite a while for me to achieve, isdnutils needs a lot of TLC).
I think both this one and bug 637286 both stem from the same basic error in your assumption: that the upstream branch is always merged into master. isdnutils is a good example where this will not be the case. Ubuntu has gotten ahead of Debian and thus HEAD of the upstream branch is not yet merged with master (I expect that to take quite a while for me to achieve, isdnutils needs a lot of TLC).