My personal favorite experience relating to this was buying some ice cream with nutritional information by the milliliter, but with serving size by the gram...
Here it's nutrition information by the gram, serving size by the gram, packaging by the millilitre.
The only way you can compare the relative value of different ice creams is by using the serving size and # servings info from the nutrition panel to calculate the grams per package. (Or even better, comparing g/fat per package because that's where the value is).
Yeah, it was something along those lines. I don't remember the exact specifics. I don't really understand why that is. I guess it's because they're copying and pasting nutritional information from the tubs where it's more properly measured by volume. But one would think that regulations would require the same units for serving size and nutritional information. Or at least the same type of unit (mass/volume).