Eileen is correct (as usual).
Here's a little background information to help you understand what's happening.
Guitar MegaVoices use the uppermost notes to play special effect sounds like fret noise, slides, etc. Check out the MegaVoice maps in the Genos Data List PDF, page 15. Pay special attention to the two rightmost columns. These columns describe what sounds when notes above C6 are played.
The MegaVoice arpeggios are designed to work with guitar MegaVoices. They use those special notes above C6, which is why they sound so lifelike.
If you play a MegaVoice arpeggio through a regular voice (non-MegaVoice guitar), those "echoes" you hear are the notes above C6. Since regular voices don't have effect sounds in C6 and above, it sounds like someone is plinking away on tinny high notes.
Hope this helps -- pj