It is also very much fight specific. If you have target swaps often then DF falls behind SS. If you can sit in one place and not have to worry about many other mechanics or you know how to manage uptime and can keep your damage doing then DF may pull ahead. SS is also proc reliant, whereas DF is not. There are many factors that come down to if one class or spec is better then another.
In a perfect world, DF/lethality tends to pull higher numbers than SS/Marksman but it is variable depending on the fight mechanics. SS/Marksman is much better at front-loaded damage and can target swap at will. Energy management is also easier and more consistant. Parsers don't show this, but it is a much better spec defensively with some nice talents to improve survivability.
DF/Lethality is heavily penalized in fights that require frequent adds, target swapping, or a lot of weaker mobs. DF/Lethality also sucks at CC as the DOTs break them all the time (more of a PVP concern than PVE). It has no (normally) talented defensive abilities and is very much a glass cannon.