Change to APEC Business Travel Card Length of Stay?

I'd been certain the duration of stay for an APEC was 90 days, it now appears I'm wrong (and 20 days over my apparent visa expiry). The strange thing is that I notice historical entry stamps from HK show a 90 day duration of stay, but it was reduced to 60 days as of approx Feb 2013 (when they started stapling a piece of paper onto my passport with entry details instead of stamping). I'm wondering if something similar may have happened with the duration of stay for mainland as well?
Bit of a bind, I'm based in China and I don't want this to create issues for me renewing my visa (going to border tomorrow to see what happens). Any input appreciated.