It's not just false modesty or pathological perfectionism: it's knowing what is possible, knowing how much better one could have done something ("could" in real-world, practical terms, rather than "could" in theoretical, "given infinite time and resources" terms), combined with a feeling of despair that the person to whom you're delivering/reporting on this thing doesn't know and/or doesn't care that it could be done several orders of magnitude better.
Also, is there a word for the lazy rhetorical device of broaching a subject with "Is there a word for..."?
