Yes, this keeps cropping up too frequently for it to be wrong now.
I agree with your theory. Mika has said that his Dad wanted him to also be called Michael, as is not uncommon in American families/dynasties, whatever.
But he said his Mother wasn't keen at all on the idea of Michael Jr. ,so they settled on Mica. But I wonder if his Dad got his own way officially , ie on his birth certificate, but his Mum got her way by insisting he be known as Mica right from the start. It makes sense.
That's my interpretation/guess.