How come one type of DC motor could have different resistances?
That's not a 'DC' motor, that's a stepper motor. Check out the 'step angle' and 'pulses per second (PPS)' columns.

The current taken at the rated voltage is inversely proportional to resistance. You get more torque for more current and therefore power consumption. 

Changing the thickness of wire that the motor stator is wound from (which is the difference between these various models) is a cheap way for the manufacturer to provide a range of power and torque options with the same basic design of motor.

To choose the motor for you, decide on your minimum torque, then choose the lowest power consumption motor that will deliver it.

If you chose a motor, but it turned out late in the product development that you'd underestimated the torque needed, then you could move up to the next more powerful motor very easily, without a mechanical redesign, or even reduce the power consumption if you found you'd been too generous to start with. A common pattern is simply to prototype with the most powerful option, then trim consumption down later once you have several real systems to test.