To your definition of inventory: "a page view plus a slot available on that page which we could sell an ad against", Google does not sell an ad display, they predominantly sell clicks on ads and therefore Google has a key incentive to display ads that get more clicks per display, hence the importance of CTR (click-through-rate) because they will get more revenue for the same page views. I am sure you know that, but I think it's worth mentioning. So I would define inventory as "a page view plus a slot available on that page which Google can use to maximise revenue".