Not sure if I interpret that correctly... do you mean you only need to live in China 3 months a year to qualify for permanent residency? Or the opposite - not staying out of China more than 3 months a year?
Only need to live in China 3 months a year to retain the greencard. I did a double take as well, as I was 'sure' that the documents I read last year (when I applied) said the opposite. Certainly not complaining, though.
I'm pretty sure the green card just allows you to live in China. It doesn't oblige you to. It means you can come and go as you please.