Inner Resistance of TRIAC
When in any doubt, read the datasheet! Fig. 12 gives the (typical, not guaranteed) I/V characteristics.



From this you can see that the triac in the on-state for currents >= 200 mA behaves like a constant-voltage drop of 0.7 - 1.1 V (temperature dependent), plus  a series resistor of ¬ 0.33 Ohm. This is the behaviour that you should expect.

(some of) You experiment uses very low currents, which is not what this component is designed for. The latching current can be as high as 20mA, hence the component should not be used below that current.