gp:port-string-height on Mac

gp:port-string-height returns very small numbers on the mac (I've seen
this consistently on various macs with OS X ranging from 10.2.8 to
10.3.9), only about half the pixel size I would guesstimate myself.
Is this a feature or a bug...? I think I can live with this by
multiplying up the height, but if it is a feature, I'm curious what
the number really measures!
-- 
  (espen)