Explanation for Arpeggiator AUTO1 and AUTO2 setting

[fuxoft]

I cannot grasp what is the meaning of "AUTO1" and "AUTO2" settings in the Arpeggiator. I understand that arpeggio melody is in fact a short MIDI sequence (which I can hear in its original form when setting arp to "PHRASE"). But then, "AUTO1" and "AUTO2" both seem to play randomly picked notes from the set of keys I am holding down. When I press (quickly) C, E and then G, the melody played by AUTO is actually different than when pressing C, G and then E, for example. I've read the manual explanation but it doesn't make any sense to me, unfortunately...