Prepaid Electricity Rip-Off

I've purchased prepaid electricity for two (2) different meters in the same security complex, using FNB mobile app, on the same evening. One with R150, and the other R250. The first (R150) yielded 91.40 units (provider ESK). The second (R250) yielded 91.80 units. This is a complete rip-off (for the second meter). What is the logic behind this?