You are not as hungry as you think you will be.

One of the downsides for me of buying lunch out before entering the office, with the plan not to leave the office until end of shift, is that I usually buy more food that I eat at noon, grossly underestimating how heavy the food is, and/or grossly overestimating how hungry I'll feel in 4 hours.

Then I have to bring the extra food home with me.