There are a couple of reasons why this could be.
1) Zivity takes 5%, plus credit card fees. Currently, credit card fees are 2.9% + up to 30¢ (or less than 30¢, depending on the transaction) per successful charge.
2) A backer may have signed up after their monthly rebilling date, which means that their pledge, just for that first month, is pro-rated. For more information, see When do backers billed and when do I receive that money?
3) A backer changed the pledge amount after their initial pledge was processed. They will be charged the new amount next month and subsequent months.
4) A backer's credit card declined for any reason.