Telkom deduction on our loaded airtime

In SA data is very expensive, i've been with telkom network for the past 6 years, i never had problem i buying airtime and convert it to data, but this year i think Telkom is reaping us of our airtime, most time when i buy airtime (e.g R50) and want to convert it to data @R49 i will find out that R10 or R12 has been deducted from my initial airtime. Last night i bought R100 airtme and when i wanted to buy data for R99 was suprised to see my balance at R80, i have a problem cos its not easy to get hold of them, they changed the whole platform of reaching their customer care ,tried even beforè several time couldn't get them, please tell us what we are paying for...