It is ok to appear smart…
Being polite does not mean that you have to conceal your intelligence with appearing ignorant or naive.
Just like several people suffer from over-confidence bordering on arrogance, several others suffer from low self-confidence or low self-esteem.
It might result from an attempt to make the other person feel good or fear of appearing cocky; regardless, every organization has really capable people deliberately taking the back seat or presenting a humbler image of the self.
Just as cocky people are avoided, overly humble people could end up being ignored in the corporate set up. While this is definitely not good for that person, it may also be detrimental to the organization.
Everyone wants to work with smart people. It is perfectly acceptable to appear as smart as you are.