The cutoff frequency of an RC filter is determined by the total resistance and capacitance at the output.
With R = 100 Ω and C = 3.62 μF the cutoff frequency is ~440 Hz. When you put 100 Ω across the capacitor the total (Thevenin equivalent) resistance is now 50 Ω, so the cutoff frequency rises to ~880 Hz. You see less dynamic error because the filter is attenuating the higher frequency components of your signal less (but it also attenuates higher aliasing frequencies less, so the sampled signal may be less accurate).
This loading effect is one of the down-sides of passive RC filters. To minimize it the load resistance and capacitance needs to be ~10 times higher than the filter's resistor and capacitor values. For a 2 stage filter they need be ~100 times higher and so on, which quickly becomes impracticable.
One way around this is to follow each filter stage with a 'buffer' amplifier which has high input and low output impedance. The buffer amp's output can be fed back into the filter to sharpen the response. A popular configuration is the Sallen Key 'active' filter, which provides 2 stages with a sharper cutoff transition than 2 passive filter stages.