Resting-state networks (RSN) functional connectivity has shown to be temporally dynamic in the brain. Also the correlations between infra slow fluctuations in electroencephalography (EEG) and blood oxygen level-dependent signal have shown dynamic variability over time. Here, we used simultaneous EEG-fMRI with ultra-fast magnetic resonance encephalography to study the link between the variations of these correlations and variations in RSN functional connectivity. The results suggest that the correlation strength is markedly linked to the strength of underlying functional connectivity. This leads to low correlations when averaged over a long period, high momentary synchrony can be reached due to intrinsic RSN dynamics.