from a quick glance, it looks like a eg8010 based output, they do have a adjustment on the output , 200 kohm with a 10kohm buffer if it follows the datasheet example... google it, lots of documentation..
Personally , I'm think extra noise is creeping into the feedback loop causing it to increase the output voltage, these chips are pretty rock solid when used correctly.. ether on the boost converters ( by pic, 4 in series) as stated earlier or in the actual " ac output" ... but without a significant amount of extra data, its all guesses ...