I've noticed the same effect. In the past (always using Nvidia cards) I kept trying GL_POLYGON_SMOOTH and I thought I was implementing it wrong because the AA looked so crappy. I finally gave up and just went with the export variable for FSAA in hardware.
I always assumed that the openGL would be strictly software AA and not take advantage of whatever AA algorithms have been builtin to the hardware.
Can anyone discredit or support my assumption?