I have seen it in another Eastern European country - the sum of all mobile subscribers each operator claimed to have, exceeded the population by 50%. I attribute it mostly to marketing tricks like counting pre-paid cards that have not been explicitly canceled towards the general tally of active users. In general the operators try to appear to have more subscribers in order to entice more new subscribers to choose them.

I seriously doubt that the telecom regulatory bodies in these parts of Europe pay much attention to the coverage/subscription rates claimed by the mobile operators' marketing departments.

Of course some of the people have more than one SIM card, but with at least 3 major operators this simply doesn't add up.

EDIT: added the sentence about the lack of regulation