In some electricity invoices the evaluation is "real", while in others "estimative"; why?

When the evaluation heading of the invoice is marked "Real", the electricity consumption has been calculated in compliance with the real indications of the meter. When the invoice is marked "Estim", the electricity amount has been determined approximatively. The network operator's personnel reads monthly the measurement device for invoicing purposes. If the network operator has no access to the measurement device, it is entitled to issue invoices based on estimative calculations on the electricity consumption for no more than 2 months.