Rambler 20-Ounce Stackable Cup

I wanted a 30 oz stackable like my others but it seems they've quit making those in seasonal colors. I thought I'd go ahead and try the 20oz because the color was cute. When it arrived it just looked so shockingly small. I filled it with water at my fridge which counts ounces as it fills and this 20 oz cup only holds 17 ozs if you want the lid on it. I double checked by pouring it into measuring cup and 17 oz was accurate. The only way this thing is ever going to hold 20 oz of liquid is without the lid and filled so full its almost running over. I'm not sure how they're allowed to advertise the product this way...why not just call this a 16 oz cup at this point? Now I'm questioning my other cups from here and what they actually hold.