Delta V question!

I am testing the theoretical delta v of simple rockets from Kerbin, but I am not getting the same results as when I blast off!

My simple rocket consists of a Mk1 Command Pod, Mk16 Parachute, RT-5 Solid Booster, and 4 Basic Fins. The total mass equals 2.5t full, 1.4t when empty. My engine ISP (ASL) is 140, so my Delta V should equal ~796 m/s.

I launch my rocket, set maximum thrust, and turn SAS on. I launch straight up into the air, but my maximum velocity from the surface only reaches ~560 m/s!

Am I not understanding Delta V correctly? Does the atmosphere have additional drag forces that I am not considering? Why is my perceived Delta V not equaling my theoretical Delta V?

Edit: Oh wow. Of course I would forget about gravity! This makes total sense now, thanks guys!