Hmm... well, the short short version is:
There are many people searching, and note ranges are found and confirmed by multiple finds and found to either be sheet replacements, or Single Note replacements (the latter usually being much harder to find and harder to confirm, therefore commanding a higher price).
A little longer answer is:
Sheet replacements example:
Suppose the entire sheet xxxx668 was removed. Well, for all 45 bricks that create the sheet, the note xxxx668 would be missing, and if replaced by pre made stock, would contain a "sheet insert".
SNRs are different in that sister bricks may not have replacements in the same spot missing. Implying that an entire sheet WAS NOT removed, thus negating the possibility that the replacements would be "sheet replacements"
Hope this helps--