You should notice that this is about appearances more than reality. Ubuntu appears to many users as one of the major distributions - the often-quoted top-three is Red Had, Novell and Cannonical. That is (and should be) surely something you can be proud off.
But this leads to people also expecting Cannonical to be in the top-three in terms of upstream contributions. And no matter how you put it, it is not.
This discrepancy tends to be very annoying to other organizations who are less visible, yet do more in terms of code contributions.
You could say Cannonical/Ubuntu simply does a better job at advertising its work. This is not in a small part due to the way it communicates and works - a rather revolutionary way (which has been copied by the Fedora and OpenSuse concepts). That's a good thing, don't get me wrong, but it creates a sense of unfairness in some minds, and rightly so.
A good solution would be to communicate more honestly about the amount of contributions Cannonical does. The general public will hardly notice it, and it won't hurt Cannonical in terms of marketing, but it will alleviate the percieved unfairness.
At the same time, Cannonical should obviously do whatever it can to increase upstream contributions. Putting the upstream work in numbers somewhere on the Cannonical/Ubuntu site would help in this regard, as it would make it one of the priorities in the Ubuntu community (and make the whole discussion more transparent at the same time).