Aeris is what the translators thought Earisu was supposed to be, because "su" can be th or s among other things.

The true origin of Aerith is mentioned in ultimania as a near anagram of the english word EARTH. IN other words, Aerith and not Aeris. The translators did not know which it was supposed to be, and took an educated guess that it was the latter.

Square do not call her Aeris anymore. Not in Advent Children credits or crisis core or any of the spin offs.