In a New York cut, 1/3 of a pizza is 2 slices.

My brother married a lovely woman I watched go to the kitchen get a plate, pull out cheese/crackers and what not, cut a single slice off the end of an 8oz block, then cut that in half and put it on two crackers. Then went through putting it all back away and walking to the dining room to sit down like it was going to be a whole ordeal to eat it. Some people are just built different. Im not going to bother pulling out a plate or even a knife if I'm not cutting up near the whole 8oz block. How many crackers, just grab the sleeve. Maybe pepperoni or sausage or w.e else I can find too.

There is some value in a standard serving size so we can compare nutrient density between products that come in different sized packages. Often labels will have two columns on the label, one for a standardized amount and another column for the entire package.