Priority is always rounded to an integer (0 or 1)

When I write the priority in an node, menu item or custom link the value that is saved changes to 0 if I chose 0.1, 0.2, 0.3, 0.4 or 0.5 and to 1 if I chose 0.6, 0.7, 0.8 or 0.9
I end up with a xml file with only 1's or 0's.
I'm using Drupal 7 with an MySQL database and IIS (windows server).

You can see my XML sitemap here:
http://www.milenio3.pt/sitemap.xml

Please help