The equivalence of the bottles is not based on the direct water content of the bottles but rather on the water scarcity impact of the consumption of 1,5l water bottles.
Since the BCome water scarcity indicator does not represent the direct water consumption in the value chain but rather the water use impact, taking into consideration the water scarcity in the different geographical areas. This is also why it is measured in m3 eq and not m3.
For you to have an idea, the water use impact of one bottle of 1,5L corresponds to 64,4 L eq, based on the AWARE methodology.