Why is CV affected by decreased price retroactively?

It is???

yes.

you mean iron sky: invasion ? that was bought through a 90% coupon and should actually be on the bundle list anyway...

Aside from bundles, games and contributor values on SG are meant to reflect current Steam prices. It's understandable people get upset if they spend $20 on a giveaway, and the price lowers to $10 shortly after. The obvious solution is to save the price of the game at the time of the giveaway. It's simple enough, but there are a few main reasons why this hasn't been done. Let's take a look.

1. Issues with consistency. Points are tied to the value of games. If prices differ per giveaway, you might see multiple Hitman Absolution giveaways, some for 50P, others for 25P. This means, every time you enter a giveaway, you'll need to check across the site to see if others are available at a lower number of points. Also, if you've entered into a Hitman Absolution giveaway for 50P, and the price drops, well, now you might want to revisit the site, remove your entry, and enter two giveaways for 25P instead.

2. Steam doesn't provide the most reliable pricing data. Prices bounce all over the place, and it's not unusual to see inaccurate data at times. For example, let's say a game is listed as $9.99. For one reason or another, Steam decides to start displaying the price of a bundle that contains the game, rather than the individual game. The price jumps to $19.99. A week later, Steam returns to the individual price, and it's lowered to $4.99. This means we have users receiving values of $9.99, $19.99, and $4.99 for the same game. The $19.99 is incorrect, so we'd have to catch that error, and readjust the values for all those giveaways. However, we don't know what hour or day the price changed from $9.99 to $4.99, so we wouldn't know which value to assign. Imagine this, multiplied by 2,000 games, and 400,000 giveaways. Also, prices need to be accurate to the minute, otherwise we'll be hearing about it in support. For example, we'll have users catching price drops, posting giveaways within minutes before our data updates, and receiving a higher value. At the moment, it's nice when we can fix a pricing error, and know it's corrected site wide. It's not perfect, but it's consistent for everyone. With individual values, I can't say with confidence our pricing data would be at the level of accuracy we need, and I think we'd have more headaches than what we have now.

Source.

As I see it CV is a relative value (despite being represented by a descrete one) and its main point is to reward the most those who are active the most AND that stay active in the long run.
Also because it's very easy to link Steam prices with value in an automated way, I suppose. :D

yeah, it's a full absurd

A better question: Why does anyone give a flip about CV? Its a flawed and heavily abused system.

Actually, it makes perfect sense.

CV is not a running total of how much you have spent. If it were, it would reflect the fact that you recently bought that one game on sale at 75% off, or received this other game in a bundle for $0.25. Instead, CV is an estimate of the value of those gifts you have contributed to SteamGifts. As the value of the games you have given decreases, so does the estimated value of your contribution (CV). This is called "depreciation" and is part of "free market" economics.