Modular units are a premium product, as such they tend to be built on the better platforms with better components that result in a more efficient power supply so they tend to be able to score higher 80+ certifications.
Its also not wrong that being modular makes a unit less efficient, the modular connector presents another voltage drop which does hurt efficiency, very very slightly. The myth was that it was equal to about 2 feet of wire, JonnyGuru wrote up an article on this back in 2006 and found that the voltage drop in 18 inches of cable was about 4x more than that lost in the modular connector, so while the modular connector does hurt a little bit, its unlikely to even amount to a single percentage point in overall efficiency.
http://www.motherboards.org/articles/guides/1488_11.htm...
80+ is also not meant for you, as an end user you will NEVER pay off the price differential between an 80+ bronze and 80+ gold unit without a ridiculous usage scenario or extremely high price/kWh in your region. It was meant for large commercial installations that pay for the power twice, once for the computer, once again for the HVAC to remove that heat. It was also meant to help reduce load on the power grid by upping the efficiency bar. I wrote up a post on efficiency myths a while ago, you can find a link to it in my sig.