Generally, this happens because users are getting disqualified from the contest, which would lower point totals.
Some other ways:
- If you changed how referral points are distributed (for example, by making confirmation emails required, mid-contest)
- deleted/disqualified users (which affects referral counts)
- or deleted/adjusted your bonus actions
Note for detecting potential cheating:
-look at the emails the contestant is referring to and if those emails actually confirm their email address. Most cheaters will refer a lot of people and those people will not confirm their email address.