A skilled premader has an average lower skill than a skilled pug. The pugger has had to adapt to better spatioal awareness, not being able to rely on the teammates doing exactly what he is expecting them to do and not having a know team composition.
Basically, a skilled pugger has had more challenges than a skilled premader. Hence, better.
First off lets do away with the myth that every pug group only plays against premades. It's propaganda used by the people complaining and wanting to paint a picture of gloom and doom. I PUG all the time and this is so far from the truth it's almost funny. Same goes with the "I only win 1 out of 25" that's pure horse manure.
A Pugger is better skilled at playing for himself and would probably do great in death match type of play but it takes way more skill to pay attention to where your team mates are and to coordinate with them in a ever changing battlefield. So please save your theory crafting, anyone who plays on a regular basis knows what I'm saying is true.