PS CS does it too but there is a solution from Photoshop Tips & Tricks.

Expanding And Reducing Selections
Have you ever made a selection like a square and tried to use the Select /Modify /Expand method to enlarge the selection? If you have, you noticed that you did not get a square at all. It will clip the corners off your selection.

If you want to get accurate results you will need to jump to quick mask mode by clicking the quick mask icon in the tool bar or by pressing the letter "Q". Once in the quick mask mode you will need to use Filter /Other /Maximum or Minimum. If your selected area is red use Minimum, otherwise use Maximum. Use the slider to adjust the amount of expansion.

Message edited on: 08/15/2005 00:53