1) The digital outputs on the screw terminals are OUTPUTS. No INPUTS. Behind there is a digital high-side switch. Only on the pinheader pins you can use the IO as input or output … and only on 5V.
2) A high-side switch is an electronic switch. You are measuring something that is “not” there. Since an electronic switch NEVER cuts off 100% … you always have some nA or uA flowing. So, if we can agree that you would never use an output without a load … even a 100k resistor will perfectly pull the output to ground. You only can read a voltage because you a measuring with a Multimeter with some Giga-Ohm input resistance.