Becasue the software isn't working right......or the chip that reports voltage (called a Winbound chip in slang even though they aren't the only manufacturer) on your board is borked. Needless to say you wouldn't be seeing windows at 0v's on the 12v
If you have a voltmeter, then you could use that to take a reading to check to see how it looks. It won't give you a real time reading, but it'll give you an idea of what your voltages look like.