This is reassuring for those optimists out there, as it suggests that people are often honest even when they don’t have to be.

Or it could be that people don't realize there's no way for the experimenters to know if they're cheating, so they're on their best behavior until they become familiar with the system.

I think this is true in a lot of domains (jobs, dating, etc). In the beginning, people follow the rules. When they're comfortable with the ins and out of the system, they start gaming it.