0-20V ADC 16-bit res. gives only half the range

I am using the 0-20V ADC 16-bit resolution and when I read the voltages with an Arduino nano, I get half the bit number. For example, when I hook up my 20V power supply to the ADC, I get a bit number of about 30,000. Shouldn’t I be getting the full 65536 range of the 16 bits?

The MCP3428 ADC is 16 bit but it can be used in diff mode as well which means its 16 bit range will be between -32768 to +32768
In the 0-20V we are only using the +ve side.