gpy v2.1A exp board P19

[kjm]

P20 on the gpy moves between 3v3 & 0v on the V2.1A expansion board with

p=Pin('P20', Pin.OUT); p.value(1)
time.sleep(1)
p=Pin('P20', Pin.OUT); p.value(0)

but if I substitute P19 for P20 the voltage swing on that pin is only 3v3 to 0.8v
Does anybody know why?