The transient response of parallel RLC circuit
During the initial transient it's quite hard to measure/compute the real ringing frequency compared to the more steady state scenario. As with all 2nd order filters (LP, BP, Notch and HP) the ringing frequency (damped resonant frequency) and the peaking frequency can be slightly different and, these can both be slightly different to the natural resonant frequency (always the highest).

The natural resonant frequency of this circuit (or a series LC) is: -

\$f_n = \dfrac{1}{2\pi\sqrt{LC}}\$

The damped (or ringing or transient) frequency is: -

\$f_d = f_n\sqrt{1 - \zeta^2}\$ where zeta is the damping factor and for a parallel RLC circuit is: -

\$\zeta = \dfrac{1}{2R}\cdot\sqrt{\frac{L}{C}}\$

See this wiki page for further clarification.


  Is there any physics behind this?


There's always physics and math behind problems like this.