Corn sugar addition at end of boil question

Here is some history on why I have a question. A month ago I brewed up NB all grain Houblonmonstre Tripel IPA. The final gravity was suppose to be 1082, and I got 1076. No big deal on being slightly off but I thought I would do the same kit again and try to get closer to the potential starting gravity. This time I checked the boil gravity throughout and finished the boil right where it needed to be at 1064 without the corn sugar addition. So I added the corn sugar at the end of the boil and whirlpooled it for a minute or so and let it sit for about five minutes before chilling wort. I measured the gravity after chilling and it was only 1071.

So here's the question. Does the sugar not disolve well enough at flameout and do I need to give it more time before starting the chilling process? I just can't think of any reason why an addition of two pounds of corn sugar would only raise the gravity by 7 points. Any suggestions would be appreciated. Cheers

Corn sugar has an extract potential of 46 points per pound per gallon. 2 pounds in 5 gallons will yield 18.4 SG points. The only thing I can think of is measurement error. How do you measure your gravity, and was this an extract or all-grain batch?

This was an all grain batch and I used two different hydrometers that I purchased from NB and got the same results with both. My wort gravity was spot on at 1064 prior to corn sugar addition. I was just wondering if I just didn't mix it up well enough or let it steep long enough in order for the corn sugar to boost the gravity. I'm not even sure steep would be the correct term. I don't feel it's an issue of the hydrometer.

Yeah, it's probably just incomplete mixing. I wouldn't worry about it and assume the correct OG per the NB instructions.