ESD/overvoltage protection with DM1231-02SO TVS array
Your interpretation is correct (note that the datasheet under Features (page 1) indicates a 12kV rating on the OUT pins vs 4kV on the IN pins which makes the in/out characteristic non-symmetric. 

The equivalent schematic looks suspect to me, though- check Texas Instruments TPD2S017 part, which is basically the same thing (with the in/out nomenclature irritatingly reversed).  

The TI part has slightly lower ESD ratings, but makes no mention of a 12kV/4kV difference between the two ports.  It does have a brief explanation of the reasoning behind the two stage ESD filter as does their slya014a document, but in a nutshell, the resistor/2nd stage diode forms a (relatively speaking) low pass filter with the diode capacitance, providing a little more protection from ultra-fast spikes (in comparison to USB signal rates anyway).

You may want to contact both of the manufacturers to get clarification on those datasheets.