We take the initial DWITHIN query and transform it into a bounding box to do a quick, coarse pull of data around the point.  This avoids performing the more expensive orthodromic distance calculation on every point in the table.  A second pass than runs the orthodromic calculation on a subset of records returned by the first BBOX query.

The other big problem is that DWITHIN only works on the underlying data units (in this case decimal degrees).  It's not possible to draw a circle on the earth in decimal degrees with constant radius that is also constant in meters.  Combining the BBOX and DWITHIN ends up yielding a good balance of not dropping any obs but also not including too many false positives.