When you increase the cost in the Product master, your inventory is now worth more so the Journal Entry is to reflect this change.

As we explain in the documentation at https://www.odoo.com/documentation/18.0/applications/inventory_and_mrp/inventory/product_management/inventory_valuation/inventory_valuation_config.html#costing-method - "If the value in the Cost field on a product form is changed manually, Odoo generates a corresponding record in the Inventory Valuation report."

If you want to use a different account, you can follow the method explained at https://www.odoo.com/documentation/18.0/applications/inventory_and_mrp/inventory/product_management/inventory_valuation/using_inventory_valuation.html#update-product-unit-price or you can set an account on the Inventory Adjustment location.