Inventory is the only term in the balance identity that accumulates. Production and consumption are flows measured over a period; stocks are a level, and today's level is the sum of every imbalance that has ever occurred. That makes inventory the most informative single fundamental series there is, and also the one most often over-read.
The market cares about stocks for two distinct reasons that are worth separating. As a buffer, inventory determines how much disruption the system can absorb before price has to do the adjusting, which makes it a substitute for spare capacity. As a signal, a change in stocks is the observable trace of an imbalance that was otherwise only estimated, which makes the weekly print the closest thing the market has to a measured read on whether supply exceeded demand.
Coverage is deeply uneven and this shapes everything. US inventories are measured weekly by a statutory survey and published on a fixed schedule. OECD inventories are compiled monthly with a lag of about two months. Non-OECD inventories, which include the largest growth in consumption anywhere, are barely measured at all, and Chinese stocks in particular are not published in a usable form. So the best-measured part of the world is a shrinking share of it.
Level, change and position relative to normal are three different readings. The level tells you about the buffer. The weekly change tells you about the recent imbalance. The position against a seasonal band tells you whether the level is unusual for the time of year, which is generally the most useful of the three and is also where the seasonal band caveats apply.