Maybe they're smarter and more knowledgeable than the average trader?
There's nothing about being a Congressperson that lends itself better to the kinds of knowledge or intelligence that would increase the chances of betting well on the stock market...
...except the knowledge that comes from a) knowing what legislation will be ahead of time, and b) getting "paid" in insider knowledge from companies that want favorable treatment. (And possibly other kinds of insider-trading-type knowledge; I'm not particularly familiar with the specific methods they use.)
How do you know that? Their outcomes indicate otherwise.
They're clearly in some regards quite exceptional people.
In any case. I'm not saying there is no insider trading going on in Congress. After all, there are hundreds of representatives. Of course some are greedy idiots, and some even criminals. But the majority, and I believe thats a large majority, are honest people who care about the people they represent.
This is assuming the consequent.
There are two possible explanations being proposed for the observed outcome "Congresspeople do better than average at the stock market."
I am proposing the explanation "they are engaging in insider trading," and proposing evidence to support that.
You are proposing the explanation "they're just better at it", and giving as evidence solely the outcome we are attempting to explain. It's circular.
A congress person is able to constantly make decisions with imperfect information and be successful doing it, one can imagine that skill could give them an edge when trading.
But the reason I don't agree with it is because they're using 3rd part managers. It's far more likely that they're advising.
However, the way to put this to bed is to compare their accounts with those of others being managed by the same team. do they out-perform, under-perform, or roughly match those other accounts? There's your answer.