The Reputation tab of my user profile looks like this: However, according to the Help Center, a user may only earn 200 reputation a day.
So why did I earn 300 reputation in a day?
Certain reputation-gathering actions are exempt from the 200 per day cap.
The most common one is the award of bounties. Any bounty received is not counted towards the 200. However, in your case, this isn't the reason.
You had a question migrated here from Superuser:- Why did SMARTDRV have to be started with /x when DOS started?. This had already acquired reputation on Superuser:- Why did SMARTDRV have to be started with /x when DOS started?.
When migrated the reputation comes with it, and doesn't count towards the daily cap on the target site.
Migration is best explained on main meta:- What is migration and how does it work? that post is also the guide for moderators on when to - and not to - migrate a question.