I don't know about Greece, but many countries do not extradite their own citizens to other countries, and include the provision that they can refuse extradition of their citizens in their treaties. France is one such country, hence Roman Polanski. Greece-Canada treaty has this provision too, so I suspect Greece is similar.
> The Requested State shall not be required to extradite its own nationals. Nationality shall be determined as at the time of the offence for which extradition is requested.