Voltage Divider not functioning as expected
Probably this coin thing has a open collector/drain output with a pullup resistor.  In effect, the output impedance of the coin thing is high enough that your divider is loading it and pulling the high level lower than the open-circuit value of 5 V.

From the voltage levels you mention, it sounds like this device has roughly a 10 k&Omega; pullup, or possibly just a 10 k&Omega; resistor in series with the output as protection.