In order to find out, measure the currents, flowing through R1 and R2.
The current you measure is 9 mA at each resistor. According to the Kirchhoff's Current Rule, the total current through the parallel circuit is 18 mA.
Imagine now the parallel circuit as a Black Box. Across the Black Box between the points A and B a voltage of 9 V applies. In addition, a current of 18 mA flows through the points A and B. . Therefore, the resistance of the Black Box according to Ohm's Law is 500 Ω.
This is the result you have already measured and calculated using the formula. However the parallel circuit has another important feature...