Why significant change in permittivity can be observed for parallel plate capacitor? (multiple dielectric plates)

Question

I have done the measurements of paralle plate capacitor with HM8018 LCR meter to measure capacitance and later, with known parameters such as surface area and separation distance, calculate permittivity of the dielectric.
I have two copper plates as electrodes and multiple 4.5mm thick dielectric plates. I have done measurements starting with one dielectric plate up to 9 plates.
When calculating permittivity, it shows significant increase in permittivity with added dielectric plates (2.13 with one plate, 8.69 with 9 plates). I have found that it has to do something with the plate count (possibly some polarization effect) but wanted to ask your opinion on this as I couldn't find such problem discussed when searching for it.
Hope I explained the problem good enough to understand, and I would appreciate if anyone could help me with this.
Measurement table:

Richold · Accepted Answer

You have not subtracted out the cable+stray capacitance, so the capacitance reading on the meter has a constant offset. Here is a plot of your data, showing the extrapolation to the plates held infinitely far apart, which should result in a measurement of zero capacitance on your meter:

Correct answer by Richold on February 25, 2021

Why significant change in permittivity can be observed for parallel plate capacitor? (multiple dielectric plates)

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