Also, upstream may have different concerns. For example, they want to be built on various versions of the distribution while Debian will usually only target unstable.
I am upstream for some of my packages and I don't provide the same debian/ directory upstream as I use for Debian.
Now that we have faster moving packages and especially where we have packages that depend on external systems (like cloud services), I think more and more Debian Developers need to be making use of backports to ensure that stable versions remain up to date.
I've heard time and time again that there are issues with Debian's version being out of date, but this doesn't have to be this way, at least not by policy. If there is a backport maintainer (doesn't even have to be the same person working on unstable) then the latest version can be installable within a stable system giving you the latest and greatest of the applications that need it with a stable base underneath.
I am upstream for some of my packages and I don't provide the same debian/ directory upstream as I use for Debian.