Lennie's theory is what I would be suggesting as well. If you get a serious efficiency bump from a "mash-out", or dextrinization rest" it will come mostly in the form of unfermentable sugars and shift the sugar profile in the wort toward being less fermentable, If you were to test fermentability before and after the dextrinization rest you would notice a decrease in fermentability but if you determine the actual amount of fermentable sugars you'll see that it did not decrease. It may have increase a little, but not as much as the unfermentable sugars.
If you don't get an efficiency boost from this rest you most likely converted all the starched before that rest and there will not be an increase in un-fermentable sugars. Just a slight increase in fermentable sugars wich does show up as slightly better attenuatuion.