The Big Deal - Where is my data that I am supposed to have on the 1st day of each month??

I took out a contract with Telkom in August 2018. SMART BROADBANK WIRELESS 20GB. This was The Big Deal" giving me 40GB Anytime Data dn 40GB Night Surfer Data. I hardly use 10 (TEN) GB each month with approx. 40+ GB remaining at month end. However - EVERY MONTH - on the 1st day of each month - "missing" GB are reflected, i.e. the new month reflects that I have already used data !!!! THis month ( at 04h56 1 Dec 2019 – 9.63 GB has been "used" ????? How is this possible? I have visited the Telkom office in Eastgate to query this - no one has ever come back to me. The service is shocking ! On the 1st day of every month I should have 80GB available for use. This is never the case. Please could someone explain the "usage" of data on the 1st day of every month - before month end I check the data availablity and it is always approx. 60 GBs left