Power rating not equal to voltage rating times current rating
Looking through the photos on the site you linked, it shows that a 5V 2.5A USB power output plug is included. This means that in order to power both the LCD and that USB power output reliably, more than the 5W which the LCD (mostly its backlight) consumes must be available.
The USB output would be 12.5W, added to the 5W of the LCD portion, you could potentially consume 17.5W. The extra 6.5W would be for inefficiency and overhead.