We were told when my son was tested that his scores were very likely an under-estimate of where he's really at. We are meant to go back and get him tested in a year or two to see where he's at.
For a child that has any learning disabilities or behavioral issues, or any signs of illness the day of the test, you can basically consider their test of be a 'basement' score... they are for sure at least at that level, possibly higher.
I haven't called to schedule DS to be retested this summer yet... I'll likely do it the summer after, as he's doing so well in his new school. The scores in and of themselves aren't particularly important for us.