Bad lines? I have old 2 stroke oil containers that I got from my folks after they sold their boat and they have marks on them and 5 quarts fills beyond where the 5 quarts normally was in the jug by just a teeny little bit and is just at the level of the filler neck of the bottles.
Those lines are supposed to be a rough estimate and if the bottle expanded(it's plastic), there will be more volume towards that 'bulge' in the plastic.
It's like measuring 150cc with a 300cc beaker... A scientist would rather pick up a graduated standard to measure if accuracy was required.
...Or who knows maybe you have an oil manufaturer to yell at.