Lowest Q factor than expected for a LRC paralel circuit
I'm not sure how you have implemented that RLC parallel circuit (simulation or physical board), but the quality factor you've got is probably due to non-ideal behavior of some components.

For example, do you know the parasitic resistance of your inductor ? Maybe it's not approximately zero, which means you'd need to consider one more resistor in your circuit diagram. In this case, new equations would have to be deduced.