Seeeduino - maximum input voltage
That’s not so contradictory but quite an usual specification. 

The max. rating (20V) is where your device will get damaged with high probability. 

The nominal input range 7-12V is where it operates without restriction. 

The range in-between, i.e. 12-20V is not recommended because it has some (internal?) restrictions. In the present case I assume there is an internal regulator that produces a lot of heat with higher voltages. So you might be able to operate it at 19V, but it would get very hot and this will limit the lifetime of the device.