Inconsistent amounts charged

I switched to an annual subscription in 2015 paying the full amount upfront, but have regularly been asked to make additional payments specifically in December months. Since December 2014, I've paid R1391.92 for the EasyView package (R29 per month). For the 4 years since on the annual payment option, I would have had to pay a total of R1276 so in actual fact I should have a credit on my DSTV account. However, I keep receiving SMS's asking to pay in odd amounts and the self-service website shows I must pay in R390 (R42 more than the month-to-month payment option). What is going on with their system and how do you know what/when you actually owe them?