Below is a look at their peak digital positions each year GAON has had a yearly chart, among the other groups in their rank.
Tier 1, 2 and in Between
Kara is seen as falling while Sistar is rising. F(x) had one high point while SNSD had one low (although Twinkle was a subunit song it was their only 2012 release). No one can predict how I Got a Boy will do and the Boys was released late so maybe it'll be an anomaly. 2NE1 stays surprisingly consistent particularly considering how many songs they have charting. Kara may fall from this rank if album and digital sales continue to decline.
Each of the groups have had a yearly #1 or in the case of BEG and Wonder Girls created a national craze. Perhaps #1 is a bit of a curse since a few groups seems not to do as well anymore, even SNSD has seen a small drop after being considered #1 in 2009 with Gee. BEG may fall from this category, since they are not active together or doing as well, but they were always judged by a different set of standards so who knows.
You can see the sort of area the groups in this tier peaked in, 20s to 40s. They had hits but never reached the Top 10 and while it seems like Secret just had a bad year AfterSchool has been in steady decline with Orange Caramel rising steadily. Girl's Day is expected to rise back with Expectation while 4Minute may also have suffered a bad year with lack of promotions, certainly if they can manage a top 20 this year and maintain their album sales they could move up. Rainbow may also move up into this category if their promotions this year are successful.
Of course album sales are also part of the equation. These numbers are what most people expect a regular album (not repack or remix ect) to sell overall from each girl group based of how they have sold in recent years. If the group sold more or less than this people would be surprised/would show a change in popularity or a particularly good album/promotions. Numbers set for 12 months since going on sale.
T-ara's has changed since the scandal of course, hence their drop on the rankings but there is no way to be sure what their post-scandal numbers might look like. Usually girl groups rank higher digitally and digital sales are more important which is why Sistar is way above 4Minute with half the album sales expected (also lots of rumours about CUBE buying back their albums but that has never been proven).
As you can see 2NE1 and particularly SNSD have significantly higher album sales which is another reason they are in their own group and between tier 2 and 3, album sales seem to make a big difference since digital rankings aren't THAT different. Album sales do represent the fanbase these groups have managed to accumulate so it is important in its own way. Kara's album sales may be close to 2NE1's but their digital sales are not and the gap in both is growing.
Part 2 of this article will breakdown each ranking on GAON's yearly Top 200 in graph format similar to this which might illuminate selling trends and why industry insiders ranked groups the way they did.