What a widening or narrowing balance means
On this construction the balance is almost always negative: U.S. field production runs below gross refinery inputs, so the domestic gap is filled by imports (and stock draws) while quality-driven exports still leave the country. A less-negative (narrower) gap means production is closer to covering runs; a more-negative gap means refiners need more foreign or stored barrels. COVID 2020 is the extreme narrowing, not a flip into surplus: runs collapsed harder than production, so production-minus-inputs moved toward zero even as the broader inventory identity built stocks.