Weird current at 8051 port 1 pin
This is an informed guess rather than a definite answer, but I suspect there isn't a resistor there at all: resistors take up a lot of area in CMOS and aren't very accurate. Instead there will be a small current source made out of MOSFETs and limited to 50uA. It might be non-linear to guarantee that, no matter what pulldown you connect, the input is close to one rail or the other.