My SIP date is not of the next month but of the month after that. Why?

Your next SIP installment date will be at least 30 days after the day on which you made your first payment. 

Example: If you made the first payment of your SIP on 1st March and set the SIP date as 10th of every month, the next SIP installment will be deducted on 10th April and not 10th March.