It appears that you have these relationships:
Attributes----<Subsystems-----<Lineitem?-----?Platform (---< means "one to many", ?---? means that I can't tell what you have for a relationship).
To fix this so that you see the correct data in your portal to LineItem, add another table occurrence of Subsystems linked using the same match fields as you have set up for Subsystems top Lineitem in your first relationship:
Then add fields to your portal row from Subsystems 2 instead of Subsystems.
The reason this happens is that FileMaker does not determine what data to put into a field from Subsystems from the context of LineItem, it makes that determination from the context of your layout's Tutorial: What are Table Occurrences?, Attributes. Thus you see data from the first related record in Subsystems linked to your current Attributes record instead of data from the first record that is realted to LineItem. By adding another table occurrence of Subsytems "downstream" from the portal's table occurrence, you get a reference that will evaluate from the correct context.
This is why you see the expected data for Platform as it is on the "Far side" of the portal's table occurrence from that of the layout's.