Random portion sizes

I now think of portion sizes more in terms of simply a unit of measurement and nothing else. Just because my cereal says a serving size is 39 g, that doesn’t mean I have to keep shaking the box just to hit 39 g. If I think the bowl looks good at 33g, I do the math and log the food and move along. It could simply be that 1.5 slices of cheese equated to some happy round calorie number, like 120, so that’s the way they chose to configure the label. It’s kind of stupid, but just decide what works best for your logging habits.
But yes, I have seen many very stupid serving sizes. I hate it when a container has like 2.8 servings if you weigh it out in grams, but the label says 2.5 servings. Or when you measure out something in a teaspoon or half cup or whatever, and then compare to the grams in the serving size, and it’s way way off.
For example: Vanilla extract powder: 4g or 1 tsp serving is 15 cal. To get 4 g it’s more like 1.5 tsp. So then I don’t know which one is more accurate - do I go by grams, or by teaspoon? When it’s off by 50%, who knows?

If this is in the US, serving sizes are typically determined by food and based on self-reports of how much people eat at a time and it's coming from governmental databases. So the numbers are very easily thrown off by a certain manufacturer making either different sizes of the same item or a company decided to go smaller or larger than average.

That's why you'll see things like ".5 of a bag" for a jumbo bag of chips. Not because the average person buying that particular bag only eats half of it, but because consumers typically report eating "x" number of chips and that happens to be half the bag.

In reality, we know that if someone buys a jumbo grab bag of chips, they're more likely to eat all of it than just half of it and if cheese comes sliced in certain portions, people are probably eating whole slices. But the government doesn't require manufacturers to go beyond calculating the "average serving size" overall for the type of food and that's why we see these discrepancies so often.

To some degree (less so after recent law changes in the US and elsewhere) manufacturers can choose any serving size that makes their label look good. This is used in particular by manufacturers who want to be able to "round to zero." They can claim zero calories anytime something is less than 5 calories per serving. This is why things like Pam spray, which is basically oil, can claim zero calories by specifying a serving size that is only 1/3 of a second spray! No one uses that little! A two-second spray, still short but more reasonable, is more like 15 calories.