I thought initially it must be a round number of flounces, but it’s closer to 21 than 20floz, so IDK.

What country are you in, OP?

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.

Possibly started with a different volume then through shrinkflation ended up at 618 mL

Which country?

I think the other commenter is on the right track and it’s likely a conversion from another standard unit. I would’ve said pint but a pint is 568ml.

If all those different products, with different shapes containers, have the same number then I’m not sure

But I do sometimes see things like shampoo coming in weird sizes because of the shape of the bottle

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 rather close to 1/6 of a US gallon, so if sold in a 6pk the pack would be 1 gallon. No idea if that’s the real reason but it makes sense to me.