A regency is a nation, or land obviously ruled by a Regent, however a Regent is a person like a governor placed in charge by a monarch to rule a land with powers of head of state or his own lands while he is unable. The point being, a regent rules by a Monarch's will to control his, or his empires lands so question being... Why is Cid a regent...?
I guess Square wanted to vary the titles as King, King, King, Queen gets dull but, well... ideas?