Low-Density Polyethylene (LDPE) and Biaxially-Oriented Polypropylene (BOPP) are part of two separate types of materials, Polyethylene and Polypropylene. These two materials are extracted and processed in different ways, meaning that different levels of energy are used in the making of LDPE and BOPP.
The way this is accounted for is through the IDEMAT database, where all market average values are measured and utilized. What this means is that for all Polyethylene and Polypropylene materials, there will be different levels of energy expedited for them, something that will reflect in a different score - despite one being mono-material and another mixed material.
To get more insights you can look at how the data moves through all the processes in IDEMAT by doing the following:
Go to the "Raw materials" section in your comparison, click "Show more", and then click on the little icon next to either "Polypropylene - Market average" or "Low-density Polyethylene - Market average"
Under "Data License", click "Read more", to get forwarded to this link. This link outlines the Eco-profiles that are used for determining the environmental impacts of plastics.