gives an explanation of the bill. Click on the green "i" symbol next to any item and you get a pop-up explanation. I am not sure it explains about the discount.
You only receive
the discount on every alternate bill, i.e. the one marked "ΕΝΑΝΤΙ" near the top left front, but it is a discount for a full four month period. On the back of this bill, top right, there is no calculation of the electricity consumed but there should be a single item labelled ¨Έκπτωση 15 %" followed by an amount, e.g. -13.92 on my last bill. Now look at the previous bill marked "ΕΚΚΑΘΑΡΙΣΤΙΚΟΣ" near the top left front. On the back of this bill, top right, there is a calculation of the cost of electricity consumed for the previous 4 month period, e.g. 92.79 on my last one. Calculating 15 % of 92.78 gives 13.917, i.e. the amount refunded on the next bill. Thus you receive the discount every 4 months and the discount is for a preceding 4 month period. Obviously you only get it on the ENANTI if you paid the previous bills on time.
PS I pay my bill using online banking on the day before it is due because it is not processed until after 17:00 on that day. If you paid it on the deadline day it would be after close of work at DEI so it might not be recorded as being on time. I don't know this for a fact but I prefer to be cautious.