FNB Prepaid Electricity Purchase Extreme Fluctuations

I bought Electricity for R500 on the 29 September 2020 and got 89,40 units using FNB app. I bought Electricity for R300 on the 5 October 2020 and got 163.60 units using FNB app.

I haven't used prepaid electricity in years so I am not sure if the costs fluctuations are a norm or I am being ****med.

Nonetheless I sent FNB an email with snippets of the purchases to query just over a week ago and haven't had any response.

Feels like buying electricity on FNB App is like playing the lottery. You could win or loose.