The imbalance you observe is due to economic allocation. The “copper mine operation” activity is allocated twice. Once having copper as a reference product and once having molybdenite as a reference product. In those cases, molybdenite and copper are by-products to the afore mentioned activities respectively. Allocation takes place using the financial value of the products. Molybdenite, has 70 times higher value than copper and therefore carries most of the burdens associated with this extraction. You may wish to check the unlinked (before linking and allocation) unit process (UPR) in ecoQuery of this activity and in the property tab you can find additional information regarding the price of the metals.
This situation you observe also occurs for other metals, when they are mined through operations that extract more than one metal. However, we are currently working on an allocation correction algorithm. Datasets in the future will be carefully documented, and correction exchanges will have comment, and some separate document will be published on our website for the users to be able to understand how the calculation occurs.
I hope this answer your question.
Data Analyst, ecoinvent