The counter estimates segments from the message as typed. At send time shortkeys such as {{customer_name}} or {{customer_address}} are replaced with real values, which can push the text over a segment boundary for some recipients. After sending, Sprwt uses the segment count reported by the carrier; if it is higher than the initial estimate the difference is deducted (logged as Sent SMS), if lower the difference is refunded (Sent SMS Refund). Keep messages short, avoid non-GSM characters, and keep the recommended 20% buffer in your balance.