The NASA News item NASA's Perseverance Mars Rover Gets Balanced says:
Just as an auto mechanic places small weights on a car tire's rim to bring it into balance, the Perseverance team analyzed the data and then added 13.8 pounds (6.27 kilograms) to the rover's chassis.
6+ kg of ballast out of 1025 kg according to the article is only 0.6 % and doesn't sound like much until once considers strict weight budgets; perhaps some scientific instrument like a methane detector or enhanced selfie cam or a next generation sundial or some other capability could have been added or enhanced instead of dead weight.
Question: Why did they just add dead weight (assuming that's what happened) rather than just slightly "move stuff around a little"? I'm thinking slotted mounting holes and/or spacers or risers for the RTG or other heavy components.