How to calculate internal generator resistance?
Your experimental approach is valid. Your concern that the power supply internal resistance should be constant may be invalid, subject to these potential problems:


Your power supply has an internal current-limiter.
Contact resistance and/or wire resistance affects your measurements.
Your power supply output voltage is affected by temperature.
Your power supply output has "ripple" due to charging/discharging capacitors


Some voltage-regulated supplies have such low internal resistance that they cannot provide current to a very small-resistance load. Until current reaches a set-point, it acts as an almost ideal voltage source, with very low internal resistance. Your measurements may be affected by the first three potential problems.

The last problem arises because many unregulated supplies have non-linear elements inside (like diodes for example) that charge up an output capacitor to a DC voltage. Your experimental approach assumes that your voltage source is a linear circuit. This kind of DC supply is not linear:






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