This figure is interesting as it comes up often with studies of isotopes. It assumes that there were equal amounts of, say U238and U235, created at the beginning (a reasonal assumption, but an assumption none the less) so the ratios of these isotopes still present and their known decay rates give a marker for their supposed date of creation.
All indications are that the earth is very old, certainly more than 10,000 years, and that still doesn't threaten the Biblical record or substantiate the theory of evolution.
You are wrong in two ways. First, many different radio-isotopes have been measured and the results are remarkably uniform. Second, the technique used is isochron dating which doesn't depend on specific ratios of two isotopes but rather on the more robust assumption tha the sample was well-mixed when molten.