Rewards: Why is the number of my contacts in a contest different than in my total contacts?

Contests and Incentive programs count OPTED-IN and ENGAGED contacts towards the contest.  

Simply entering a contact does not make them count towards the contest. The contact has to click on the link from the campaign or a resource sent.

Some campaigns might be excluded from the contest as well, so please check current rewards / contest requirements. 

More on Rewards: Rewards: How do I see all available rewards?