Is it possible that the 3,617 square foot number is TOTAL square footage or "total area"? Meaning, it would include the garage and any screened in lanai/porches? If this is the case, the 2,829 would be square footage under air or "living area".
I don't know if this is the case in California, but here in Florida, every listing has two numbers that represent square footage and this is the difference.
Hopefully this helps!!
Florida's Realty, LLC