If you’re on one of the standard Dibz pricing plans (i.e not on free trial), the leftover credits from your subscription don’t just disappear at the end of the month.
Your credits (possibly including those you received from referring another user) can last for up to three months, and since the credits that you have left over from the month before are only used when you spend the ones you received for the current month, this can often stack up quite a bit.
In short, your credits have a three-month date of expiration, and when charging you for your services, Dibz first uses the newest credits you have.