In 1973 Greece switched from a monarchy and a military dictatorship to a republic and a democracy. Their coins of 1973 reflect both regimes, but all the reverse designs on the republican set were only used in 1973 - though two of them, the dolphins and trident on the 10 lepta and the olives on the 20 lepta also appeared on the monarchical set, but again for 1973 only.
As far as "one year sets" are concerned, you are right of course. But (diatribe spoiler

) Greece did certainly not turn into a democracy in 1973. The country had a military regime between April 1967 and July 1974. Formally, Greece was a kingdom until June 1973. Then Georgios Papadopoulos, who had been regent or vice-king before, decided he would be president of a republic instead. (Had to do with king Constantine's attempts of playing a more active political role.) So Greece became a republic on 1 July 1973 - hence the switch from the
Kingdom to the
Republic* inscriptions. After the end of the military regime, Greece did away with the phoenix symbol.
* Most of us will use this Latin-based word when referring to a republic; in Greek however "Dimokratía" (Δημοκρατία) has the same meaning.
Christian