I got one-year EAD based on I-485.

I filed I-485 in Aug, 2007. recently, I filed EAD for me and my wife. Today, we got the cards, but it only has one year instead of two years.
Everybody is saying that EAD will give two years. I wonder why.
Does anyone have experiences with this situation? and how to get two-year EAD?

Thank you a lot!