Yes RHR (for me atleast) is pretty unstable and there can be hundreds of explanations.
When I think I might have been overtrained my RHR was at 44bpm, however when I was at my results wise fittest it rose to 48bpm to 50bpm. A sh1t sleep wil see that rise to 52bpm and an illness perhaps 55-57bpm. Then again perhaps I was actually fitter at 44bpm 