It's my understanding that it's not unusual for IQ scores of gifted kids to drop by as much as 20 points after age 8, especially for HG+. I can't find the source I read, but this mentions it:
http://www.tokyochildrensacademy.com/admissions_overview_assessing.htm
In addition to the tester, a particular test can produce much lower scores than other tests which the article also discusses.