Translation:nā mea ʻai aʻu
I believe it has to do with the difference between the 'simple' and 'complex' sentence structure found in the language.
Whereas "maika'i ka pua" is a self contained and completed sentence, where the adjective proceeds the noun. "ka pua maika'i..." Is a as of yet incomplete sentence where the adjective comes after the noun and that requires further specification.
So too could "na mea 'ai a'u" be the complete sentence simply meaning 'my food' whereas "Ka'u mea 'ai..." Be an as of yet incomplete sentence.