Any particular person getting a 20% return is not suspicious. The question is whether the distribution of returns over all senators is suspicious.

Any person getting a 20% in some small number of trades is fine. An individual getting 20% over a long period and a wide range of trades is definitely suspicious.

Or a single person outperforming over a long time horizon.

When you have enough people trading over some period of time, someone will have consistently high returns.