The Prime Minister put all of his assets in a blind trust. He doesn't touch it, he doesn't talk to the trustee, and he doesn't make any decisions about that trust. Naturally, he knows what assets he owned before handing them over to the trustee.
That said, decisions relating to companies in which he held assets could be profitable for him if those decisions led to an increase in the assets' value. That's why a conflict of interest screen was put in place. The idea is to prevent the Prime Minister from making any decisions that relate to the assets in the blind trust and that could affect their value.
He obviously only knows the assets that were placed in the blind trust in the first place. Maybe those assets have changed or have since been sold.