I believe that the title is expecting the reader to infer that the measurement is about plastic waste, not water quantity, which commonly is measured in distance.
They are obviously talking about the bottles and not the contents of them, which you'd know if you'd read the article.
or s/bottled water/water bottles
EDIT: title was changed.
The article discusses the length of the bottles placed end to end. Although I agree the title isn't clear.
What are you talking about? The title "See what researchers found when they tested a bottle of Fiji Water against a glass of tap water" is straightforward, tells you exactly what the article is about, and cites sources appropriately.