  A long discussion has been devoted to the issue of clustering of the kHz QPO
frequency ratios in the neutron star sources. While the distribution of ratios
inferred from an occurrence of a single QPO seems to be consistent with a
random walk, the distribution based on simultaneous detections of both peaks
indicates a preference of ratios of small integers. Based on the public RXTE
data we further investigate this issue for the source 4U 1636-53. Quality
factors and rms amplitudes of both the QPOs nearly equal to the points where
the frequencies are commensurable, and where the twin QPO detections cluster.
We discuss a connection of the clustering with the varying properties of the
two QPO modes. Assuming approximative relations for the observed correlations
of the QPO properties, we attempt to reproduce the frequency and ratio
distributions using a simple model of a random-walk evolution along the
observed frequency-frequency correlation. We obtain results which are in
qualitative agreement with the observed distributions.
