The basic story with extended norms is that they correct a reporting error caused by the low ceilings. The correction takes place by adding to the scores so all of the higher scores become inflated.
Without extended norms, the margin of error for IQs from like 70 to 130 might be +/-3 pts (making the margins up for example.)
But from 130 on, the margins get larger. So you get to 160 before extended it might be +/-20 pts. With extended norms the margin of error narrows for the higher scores; such that 160 might now be +/-5 pts.
Since these are +/- errors and no scores are lowered, the meaning of higher scores are decreased to reflect the full population of scores with extended norms.
Imagine two kids, one scores a 125 and the other scores 130. The 125 has some lower scores in processing speed and working memory, but also completely maxed block design and picture concepts. With extended norms (the full 26 pts rather than non-extended 19 contributes for both of those subtests) the 125 becomes a 142. Every score raised lowers the % of population at the scores it now surpasses.
Not sure if that answers your question or not.
