Prices doubled with no explanation and clueless WhatsApp helpdesk

Terrible customer service on WhatsApp (good response time, but clueless). They can't answer so they are ignoring me. Also no response on Twitter either, so going this route now: We moved into our house in November and the prepaid electricity was already filled for us for the first month. On 23 December 2021 I bought R200 of electricity on the FNB App and got 128.2 units. Again on 14 January 2022 I bought R200 of electricity on the FNB App and got 128.2 units. But on 01 February 2022, when I bought R400 of electricity on the FNB App and only got 138.9 units... that is a measly 10.7 units more for double the price!!!?? When I asked the WhatsApp helpdesk they were confused, tried to tell me vaguely that you get different amounts for different purchases, but cannot explain the unbelievable difference to me - because that is ****************! Please tell me why the cost of electricity basically doubled! FYI There are two prepaid meters in the house, this is for the second meter in the granny flat. Neither of the houses were in arrears (as far as I'm aware you can't be in arrears on prepaid).