According to
http://www.apple.com/imac/specs.html , it's 160/160. You must be getting the 170/160 figure somewhere else, but then again the panel in question does not exist on the manufacturer's page
The first theory is in fact what I already suggested, which is you're looking at a better TN panel in the 2009 20", but I don't buy the second theory that it's using a MVA. MVA does not have a contrast shift from top to bottom, which others and I have confirmed in the 2009 20". The horizontal viewing angles are very good though and nearly indistinguishable from the 24" I compared. Looks much closer like a 170/160, not a 160/160, but still the slight contrast shift from top to bottom gives it away that it's a TN panel.