You could ask to see the DEI bill, although you need to know what you are reading. If you have a meter there is probably a bill for that one meter.
Agree with Jean that you should be tactful about it though - there's nothing worse than fighting with a landlord or making an enemy of them even if you are in the right.
Meters are only read by DEI every 4 months, while bills are issued every 2 months. So every second bill is an 'enanti' which is an estimation of usage from DEI based on the previous year for the same two month period, plus taxes and TV charges (as a very general guide these extra charges are usually around 20 to 25 euros per 2 months, but vary according to the size of the property and the Dimos. Also add on 9% tax to the total unit costs).
When you receive the 'ekkatharistiko' - final bill for 4 month period -any adjustments are made.
So there is a possibility that someone who lived in your apartment last year had high consumption and an 'enanti' bill would be based on that.
There's also the possibility that, if the bill is final (ekkatharistikos) and only for your apartment, that there is a previous tenant's usage on it (e.g the 'enanti' hadn't been paid). You'd have to check the meter reading on the bill.