I think you're on the right track. My guess would be that they have a 3% tolerance (uncertainty, idk) with filling so they fill 600 ml but statistically it might be as much as 618. Putting 618 on the packaging lowers the price per liter a little, compared to 600.
This seems backwards from what a manufacturer would want to do. The concern with variances isn’t really having too much but having too little in the bottle. If you aimed to put exactly 600 in the bottle, you will sometimes end up below 600. It would make more sense to label it 600, aim for 618, and be confident that you’ll always fill it to at least the advertised 600.
In Canada we have a lot of that and I always assumed we import things from America and then just change the labels. The metric usually converts to a more reasonable number in imperial.
You do sometimes see nice round metric numbers, for example soft drinks (soda / pop) often come in 2 litre bottles.
I'm still unsure as to why soft drink bottles are measured in litres while milk is measured in gallons... A carton of milk (half a gallon, 1.9L) is almost as large as a bottle of soft drink (2L) so it's strange they haven't converged.
There's also things like the TSA liquids limit, which is defined as exactly 100mL but commonly written as 3.4 fl oz.
Do you perhaps mean 568ml? It's a pint and beer quite commonly comes in that size. Some water, like liquid death comes in that size, and I've just googled and so does shampoo.
Working at a store, you can rest assured that number will get smaller in a year or less. I've caught dozens of products getting smaller but costing the same.
It’s the Illuminati. They have to tell/show you what they’re doing for some reason. Probably has to do with that. I don’t know what it means though. And yes, I read conspiracy theories.