Pre-paid electricity discrepancy

When we bought pre-paid electricity in February, through vendor X, we got 551.6 units for R1 000.

On 12 March we bought pre-paid electricity from Capitec for R1 000 and got 383.0 units.

A DIFFERENCE OF 168.6 UNITS.

I WONDER WHY THERE IS SUCH A BIG DIFFERENCE!

Capitec say they cannot help iI must speak to Joburg City Power who does not respond to calls, emails or tweets. I asked Capitec for assistance with no avail.