Sounds to me like you're in the ballpark. FW capacity usually includes the WH, so there is 10 gal of it. Then the tank manufacturer places a size rating on their product which is completely full, no air at the top. They might even ignore the radius in the corners. Then if the fill and vent ports are in the side, as they usually are, you cannot completely fill the tank. Add a little in all the plumbing lines, and you end up with much less useful capacity. It is even possible the outlet port is in the side, which means it will not fully drain.
FW capacity is important to me, and if it is to you, I would suggest you get eyes on that tank. Note where the inlet and outlets are plumbed, and when full does it still have a lot of air at the top. FW tanks are usually translucent, so you can see the level pretty easy if you hold a flashlight against the tank wall. I cut a window flap in my corroplast so that I could easily check on the tank.