that would explain a lot since I'm very sure that Skyrim has been close to $5 before at some point.

Do free games count as sales? I remember L4D2 was free awhile back (on multiple occasions) and me and a group of my friends downloaded it.

Hmm so it was just a free weekend then.

Including people who added the game to their Steam account, but did not purchase through Steam.

Same. 

They sell well only if it comes to sales for very cheap price but not full price like consoles players pay. Most of the sales from steam are like giveaways for very small price. I bet only 20 - 30% of them are full or good price sales and rest are very cheap deals steam trick.

If the game is good, many PC gamers also buy them for full price: http://steamspy.com/app/271590

Trying to frame PC gamers in some narrow stereotypes doesn't make any sense. sure, we like to save some money on games, but:
1) devs haven't to pay royalties to develop on PC
2) they have a long record of releasing games still unstable (a thing that on consoles started only when they began having some forms of rewritable mass storage), this further incentivises us to often buy later.
Buying at better price doesn't automatically mean buying at 10 dollars or euros or 5 or less: like on consoles, PC games have many pricing levels through their commercial life, they launch between 45 and 60 ($ or €), and at that level they can be commonly found discounted by 10 to 20%, later, when still officially full priced, they can be commonly found discountedby 20 to 30%, later on they receive their first discounted edition, often enhanced and with expansions included, for around 30 $ or €, much later they receive bargain editions at $/€ 15, 10 or even less (often the case of those bundled with gaming magazines). And this for physical support.
For physical support, particularly in the past, there was the plague of late, overpriced badly localised versions so that people knowing at least basic English were often tempted to buy in USA or UK.
For example I bought Morrowind GotYE for 30 euros on amazon UK when it was still full priced in Italy.
About me, I have few games I bought with a modest discount when they were still full priced, but amongst the best of the rest I bought the majority of them when priced between 20 and 30 euros, while I bought at 15 euros or less just the oldest ones or casual purchases that I wouldn't have bought otherwise or games that I didn't know yet and happened to be worth a lot more than their price when I discovered them, lucky me, this applies for example to Myth I and Gothic I, and also to Fallout 2, I thought I wouldn't have liked its settings very much, but when I found it bundled with a gaming mag for less than 10 euros I bought it and I changed my mind, it became one of my favoured games ever. This to say that even the lowest pricing levels can benefit games bringing them new gamers that they'd never get otherwise, either because decentbbut not particularly good or good, even excellent, but relatively unknown.

😆 at those skyrim sales.

Would like to see more of the best sellers.

the developers know better than us, don't they?