pre/post boil gravity question
 

Brewed a oatmeal stout today with a target of of 1.064. Stopped fly sparging when I had 6.5 gallons pre-boil. Pre-boil gravity reading was 1.048. Boiled 65 minutes and ended up with almost exactly 5 gallons. My OG ended up being 1.070. Something doesn't seem right here.. 6.5x48 = 312/5 = 1.062, how did I end up with 1.070?

Were you taking gravity readings with a refractometer or a hydrometer? If hydrometer, did you take both readings at room temp or close?

Gravity readings were taken with a hydrometer. Pre-boil reading was temp corrected, post-boil reading was taken when wort had cooled to 65.

Volume measurement errors?

I have a wooden dowel that I use so I get exactly 6.5 gallons pre-boil, and I used the gallon markings on the bucket and ended up almost right to the 5 gallon line, maybe like 4.8-4.9 gallons.

This is why. You can throw the temp corrections out the window if they get too far above the calibration point.