Sorry, this reply is probably too late. I have a hunch it's because I'm in the US very frequently, probably more than 6 months in a year. I didn't overstay my visa either. I would just leave and come back. There's an unwritten rule wherein if you stay in the US for more than 6 months in a year, it's pretty much your place of residence. That violates the purpose of the tourist visa.
However, that didn't affect my green card. Though every time I fly into the US, I am held at secondary inspection. It's a little annoying, but they'll let you pass at the airport since technically, you didn't really violate anything.