Don't worry, it is 5K.
By default, the UltraFine 5K is set to 'looks like' 2560 × 1440, but is rendered by 5120 × 2880 pixels. The idea being that the image produced is four times as sharp than a standard, non-Retina, 2560 × 1440 screen.
You have it set to 'looks like' 3200 × 1800, and this too is still rendered by 5120 × 2880 pixels.
No matter what scaled option (e.g. "larger text" or "more space") you choose in display preferences, you are still enjoying full 5K resolution. The only thing that changes is how big the things on the screen are. 14.7 million pixels are still being produced.
Apple should make the interface more clear in this regard.