[nazgul]

Say the subreddit generally only get 5,000 page views a month, with one ad space per page. Once someone purchases those 5,000, it's gone -- they can't sell the inventory to someone else because it's all bough. Often too traffic will fluxate; so, for example, they may have estimated 5,000, but only 2,000 page views (and thus ad impressions, since one for one in this example) come in that month -- if someone bought 5,000, they'll have to stick around for 2 1/2 months, blocking other advertisers for that time. And I suspect that on many of these subreddits having even 500 impressions available is overreaching. That's a bit simplified but hope it helps.

[yalogin]

Thanks!