What divides Hydrological Unit Code areas of the same level?
I'm confused about your misunderstanding as the answer to your question is in the Wikipedia link you posted:


  Regions receive a two-digit code. The following levels are designated
  by the addition of another two digits.[8] The hierarchy was designed
  and the units subdivided so that almost all the subbasins (formerly
  called cataloging units) are larger than 700 square miles (1,800 km2).
  Larger closed basins were subdivided until their subunits were less
  than 700 square miles.[7] The 10-digit watersheds were delineated to
  be between 40,000 and 250,000 acres in size, and the 12-digit
  subwatersheds between 10,000 and 40,000 acres.[6] In addition to the
  hydrologic unit codes, each hydrologic unit was assigned a name
  corresponding to the unit's principal hydrologic feature or to a
  cultural or political feature within the unit.[7]


So, once you are outside of the 2 digit regions, HUC boundaries (or dividing/outlet points along a continuous stream) that separate an upstream watershed from one a downstream one (that should incorporate the upstream HUC using the classic definition of a watershed) are set to evenly divide up the larger (or containing) HUC.