Inverting op amp gain using high value resistors and using oscilloscope measuremnts
You don't say what amplifier you are using, or what measurement frequency.

In an ideal world, you would get the 100x gains you calculate. So the puzzle is, given your limited information, to guess what is causing the situation to be non-ideal?

If you are measuring with a high AC frequency, then stray capacitance across Rf would reduce its impedance.

If you are measuring with a high AC frequency, and using an amplifier with a limited gain bandwidth product, you might be so far down the gain/frequency curve that the gain has fallen to less than 100x.

If you are measuring at DC, with a poor amplifier, or an old/damaged/wet Rf, then there could be a resistive path shunting Rf, reducing its effective value.

It's possible to do further experiments to see which if any of these are causing the problem. Change the measurement frequency. Measure the amplifier open loop gain. Load the output of the amplifier with different load resistors. Replace the single Rf with a low impedance 10:1 potential divider and a series resistor of 10% of the value (should give the same gain, but with lower feedback impedances).

This sort of investigation is what engineers have to do all the time. Remember, that in an apparent impossibility like this, the measurement, or at least some aspect of it, is always correct. What is at fault is your assumption of what exactly the measurement setup is. 'The measurement setup' of course includes the thing you are measuring, the stimulus, the meter, the connections, and anything that could influence it. Go investigate.