Voltmeter woes

Just got duped. The digital panel-mounting voltmeter I bought which supposedly has a range of 0 to 20VDC stops measuring--with fairly good accuracy--at a little over 15V. Feed any voltage higher than that and meter's accuracy starts dropping off fast. By the time the measured voltage is 20V the meter reads less than 17V.

The exasperating part about it is I found this out only after I had redesigned a circuit so as to factor its limited range of 20V. If you haven't guessed it yet, the cheapo meter is made in China. Yes, it came in that blue and white box with Chinese characters. Doesn't come with any instruction leaflet. And there are only Chinese text on all sides of the carton. Frustrating. At least the silkscreen on the board isn't in Chinese! As you can probably make out, the pins for the connectors are marked (left to right) 5VIN, GND, GND, VIN+. The first two pins are for power and the latter two are for the measured voltage input. A check using a DMM shows the two middle pins (GND) are in fact connected to one another on the PCB. The multiturn potentiometer (blue rectangular component at the top) is used to calibrate the meter. Calibration is fine--if you don't need the entire specified range of 20V. Else, it's a joke.