Pre-boil gravity higher than post boil

Hey guys. Been poking around these forums for a while but I finally felt the need to post. I've been brewing for about four months now and have done four all grain batches.
All my other batches have increased in gravity after the boil, a result of boiling off water. The one I made yesterday did not, or at least I don't see it.

Pre boil - 6.5 gallons
1.052 @ 140 - 1.068

Post boil- 5.75 gallons
1.052 @ 80 - 1.054

I can't really think of any reason my pre-boil would be this high. My grain bill was only 11.75 pounds and I am not all that efficient, so why would my pre-boil gravity be so high?

Another thing to note is I used 1.25 oz of hop pellets, and while straining most of my wort, I had to leave maybe a quarter of a gallon in the pot because it was too thick. The pre-boil doesn't take the hops into account so I don't think this matters.

Thanks in advance for your help.

It looks like you are correcting for hot sample temperatures. I would expect there is an error in the correction. The only way to be truly accurate would to have the samples at about 60° when taking the readings.

As far as I know it would be impossible for the gravity to decrease while boiling.

Two things:

Make sure you mix your preboil wort really well before you take a sample. Also, the temperature correction for your preboil gravity @ 140 degrees is probably wrong. That scale starts to get way off at really high temps. Just cool down your sample to 70 then take a reading. I usually put a pint glass or two in the freezer then pour my preboil sample in that to cool it off, take a reading, then pour it back in for the boil. Hope that helps.

Make sure you mix your preboil wort really well before you take a sample.

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Thank you for the quick response. I think you solved all my problems. I did not mix it at all and my thief took the sample off the bottom of the pot and probably got my first runnings in it.

Also, the temperature correction for your preboil gravity @ 140 degrees is probably wrong. That scale starts to get way off at really high temps. Just cool down your sample to 70 then take a reading.

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I am about to invest in a refractometer. Hope that will eliminate the errors.