... someone experienced with gifted kids may test differently from someone unfamiliar with them...
Yes, this a very good point to be aware of, so glad you mentioned this. It is discussed briefly on a recent thread here:
http://giftedissues.davidsongifted....t_do_you_tell_your_DC_be.html#Post174222 In a nutshell, here are two great articles about testing, written several years apart:
1- Aimee Yermish, 2002, linked from Hoagies Gifted Education Page
http://www.hoagiesgifted.com/test_prep.htm2- Nadia Webb, 2006, linked from Davidson Database
http://www.davidsongifted.org/db/Articles_id_10404.aspx It is standard among all of the psychologists who regularly test HG and PG kids, to start the subtests above their chronological age. It keeps the testing from being tedious or seen as patronizing.
Beginning with questions above chronological age may be seen as one important difference from the Yermish article which describes the test as beginning with easy questions (the kind which many describe as inspiring kids to give silly answers, detracting from their performance score).
This is not to detract from Aimee's article, as the purpose of the article is for parents to prepare children for what to expect during testing... even if a tester is choosing questions which are above chronological age, they may be seen by an HG or PG child as "easy"... therefore it may be wise to prepare a child that they may receive easy questions with obvious answers... so they can decide ahead of time not to be too silly if this occurs. 
