Experimental calculation of dc motor resistance
There may be some accidental experimental error here : You may expect to measure the resistance of just a set of windings, and that should be constant regardless of the drive voltage.

However unless you have locked the shaft AND kept it locked throughout the series of measurements, you may see something different. Measure at a specific voltage, and rotate the shaft very slowly by hand (if its a low power motor : otherwise take proper safety precautions and use a low voltage!) 

You will probably see the resistance measurements fluctuate as brushes make and break contact with commutator segments. You may see the resistance halve briefly as one winding connects before the previous one disconnects.

Maybe this is what you are seeing in your series of measurements?