ST_PointOnSurface returns point on the boundary of the polygon

Calling ST_PointOnSurface with a polygon argument returns one of the points of the polygon boundary.

E.g.

SELECT ST_AsText(ST_PointOnSurface(ST_GeomFromText('POLYGON ((67 13, 67 18, 59 18, 59 13, 67 13))')));

returns POINT (67 13)

The spec says

PointOnSurface( ):Point—A point guaranteed to be on this Surface.

which might not be completely clear, but several sources indicate that "on this surface" means inside the surface.

When running the same query against a postgis instance (not through teiid), the result is POINT(63 15.5), which is actually inside the polygon.