Why might Congress have a higher percentage of Republicans then a general public

Congress has historically had more republican representation due to representative democracy.

Explanation:

The states where the population is less down south are usually republican strongholds and thus while there may be more democrats in the whole population of US that is not how the people are elected in US democracy.

In representation the population size is not truly reflected thus it is possible for more republicans to be elected as states with lesser populations get to vote for more candidates in comparison with the populous states.