Maybe they have one machine set up to fill the bottles. In one market they are required to deliver a safety margin of 3%. So they put 618 in a 600 bottle. In the other market there is no such requirement. So they write 618 on the bottle.
If by law they need to never be under the written volume, then writing 618 means they need to put 618 * 1.03 in the bottle to make sure they hit the 618ml written on the bottle.
In your example, they would write 600ml on the bottle and fill it with 618 ml to account for the machine tolerance.
Maybe they have one machine set up to fill the bottles. In one market they are required to deliver a safety margin of 3%. So they put 618 in a 600 bottle. In the other market there is no such requirement. So they write 618 on the bottle.
If by law they need to never be under the written volume, then writing 618 means they need to put 618 * 1.03 in the bottle to make sure they hit the 618ml written on the bottle.
In your example, they would write 600ml on the bottle and fill it with 618 ml to account for the machine tolerance.