not really a 20oz tumbler

bought this tumbler because they said it is a 20oz tumbler, but it doesn't fit 20 oz comfortably without spilling over and when you put the lid on it, forget about it. this is actually a 16-18oz tumbler. meaning when I go to starbucks or anywhere to order my venti latte, part of my latte will spill over or they just won't fill it up to the top because they would need to leave space for the lid. and if I'm paying for a venti, I want my venti. disappointing purchase.