Strange trend for ECG gold electrodes impedance at DC
We really don't know what's happening with no raw data and a rapidly evolving subset of information, but playing around with a spreadsheet on what we've got...

Using Vout=Vin * R/(Z+R), the published step sizes in ohms, and guessing a step value of 58200 for the last full step (from reading the second graph) we can reverse engineer the original output voltages Vout as measured by the DAQ.

Step 4 (3247) is anomalous ... was it mis-typed from 2347 ohms? Assuming it was brings it much more in line with the rest of the sequence, and the computed resistance values look like those on the first graph: 

( 28327, 30383, 32534, 34827, 37174)*, 39700, 42337, 45158, 48127, 51290, 54642, 58200

(* subject to guess on Step 4)

These correspond to voltages falling from 0.46 to 0.27 (as a proportion of the still unpublished Vin which I will subsequently scale to 1V) covering "several hundred" millivolts. Each step then varies from 17.2 mv in a smooth sequence down to 12.9 mV which is ... suspicious....

How sure are you of the "known" resistance being 22.6 kilohms?
If we assume it to be 60000 ohms, we get an approximately equal step size of about 15.5 to 15.7 mV (again, scaled to Vin=1V).

Now, if you measured a slowly ramping voltage with an 8-bit DAQ (256 steps) and a reference voltage of 4V, you would see a sequence of 15.625mV steps.

Coincidence?