Inventory (advertiser speak for "a page view plus a slot available on that page which we could sell an ad against") are a lot like cuts of meat. Some are worth a lot of money. Some are not, and end up getting sold to be turned into fertilizer or paste to infect cows with all matter of diseases.
Things that tend to make inventory worth more money:
1) Better matching algorithms. This is, fundamentally, why Google is rich beyond the dreams of mortal men.
2) For contextual ads, being matched against content which suggests immediate purchasing intent for something which has a high customer LTV, high transaction value, and/or high margins.
3) Failing that, for brand ads, characteristics of the inventory suggest a "desirable" person at the keyboard. Ideally, you're a rich white American, male or female depending on which brand is trying to reach you, and this is one of the very few ads you will see today.
4) Within this particular session, this is the first ad you've seen.
5) You're American, Canadian, British, northern European, gap here German, Japanese, big gap other first world nation, titanic gulf anyone else.
6) The page the ad appearing on is itself of high social esteem such that the ad will have the halo effect.
Inventory which matches the above descriptions can be sold to advertisers using dedicated sales representatives. Everything else is remnant inventory -- literally, that which remains after our ad sales guys had as many steak-and-booze meetings with the marketing execs at Audi as they can possibly have.
Options for remnant inventory are: a) backfill ad channels like AdWords, which makes some remnant inventory almost as valuable as brandable inventory (e.g. if I write a blog post about credit cards and you happen to read it you're very nearly as useful as someone reading a post on the NYT about credit cards), b) house ads, and c) backfill from ad networks, such as Google house ads. (House ads are, e.g., "We couldn't sell a commercial for our TV channel so instead we'll show a commercial for another show on that channel or another business affiliated with ourselves.")
So, why is your page view remnant inventory?
1) Because generic Internet news sites generate a metric truckload of page views.
2) Because your user behavior was, sorry to say, probably not seeing that ad as the first thing you saw today.
3) Because algorithmically you don't look like a rich white American.
4) Because there are very poor transactional options for monetizing someone if all you know about them is "Possibly interested in dinosaurs," so direct response marketers are unlikely to bid on that site.
5) Because physorg.com does not have a sophisticated ad sales operation which routinely has steak dinners with ad buyers at Proctor & Gamble or any other large brand advertisers.
OK, so you're remnant inventory. Why are you seeing Google+ game ads?
1) Because Google has virtually infinite funny-money to buy ads on Google AdSense at low, low prices because they're crappy backfill remnant inventory that costs them CPMs below $1.
2) Because Google thinks promoting Google+ is their overarching strategic priority right now (they're wrong) and ...
3) ... because Google thinks that engagement on social networks is in large degree driven by games (they're right)
Is remnant inventory necessarily a bad thing?
No, because remnant inventory will inevitably be generated by attempting to monetize (with ads) any website which can't afford a direct steak-and-drinks ad sales force who can charge stupid amounts of money to brand advertisers. If you perceive any value at all out of physorg.com, you should be happy that Google is underwriting it out of their massive pile of money that they get out of controlling navigation and advertising on the Internet.
There exist non-advertising methods by which you could subsidize the creation of content which you like. Many of them involve paying non-trivial amounts of money, or praying that your interests happen to align with the interests of someone who is willing to subsidize your own consumption.
Remnant inventory also enables the creation of actual value, though you're not seeing it when you see diet pill scams. For example, almost all of Bingo Card Creator's substantial advertising budget is spent on remnant inventory at 3 to 8 cents a click, and while the websites that $15k+ a year goes to are not ones you'd really love your mother to be spending time on, selling BCC to the teachers who happen to be on them does at least help children learn to read and underwrite my quirky online pursuits like writing excessively long comments.
Remnant inventory is also virtually synonymous with Google, which a multinational advertising company which occasionally produces industrial biproducts of advertising which generate non-trivial amounts of value for almost every human alive.