A couple of years ago I was all ready to go out the door for a 2-day drive to Wyoming and weighed my pack at the last minute. I found that the pack weight (on my reasonably accurate bathroom scale) was almost 2 lbs. less than my spreadsheet total.
Of course I thought for sure that I'd left something out! I went through my spreadsheet printout (which I use as a checklist) to find out. Everything was checked off! So I unpacked everything and started checking it all off again. I finally figured out that the weight discrepancy was the food. I used an average day's food to calculate the total, and that's how much the total was off. I threw in a couple of breakfast bars (which I didn't need) to help with the discrepancy and finally got on the road.
As a result, I got to my motel in Idaho about 11 that night instead of at 6 as I'd originally intended!
Now, when packing, I weigh my food bag and put the actual weight on my spreadsheet! Most of the time I trust the spreadsheet total, too.
There's probably a moral here someplace! :-)