Have you guys seen the sales of the other new albums that were out this week? They're nowhere NEAR Kuu and Kaela's figures.
You guys aren't considering this... Kaela is pretty popular now right? And this is her FIRST Best album and this is how much it's selling? Almost the same as Kumi's what? 5th Best if you're going to include ~B&L~ and OUT WORKS.
Kuu's been in a state of decline right after Black Cherry album, she's fought through a stupid scandal and releases 4 - 5 singles before each studio album. Not to mention that more than half the songs that were HITS in Japan are on ~third universe~ were already released in her highest selling album, Black Cherry, and above-average selling album, Kingdom. So I don't think anyone really cares about the best.
She has average to low sales for singles but she's still able to sell 200k for an album in a week. When she struggles to do that, that's the time I'll start worrying. Last year TRICK was the 2nd highest solo female studio album if I'm not mistaken so I would also say yea, the decline does apply for pretty much everyone unless you're in a boyband. Or Namie lol but you can still argue she only released 1 single before this album and her previous BEST. I don't know. There are many arguements you could use to give reasons.
But it's silly to say it's just Kumi that is declining. Hello, Ayu anyone? SHE was the million-selling album master, not Kumi. Oh and I wouldn't say 5 BEST's are too much in 10 years. Ayu has the same number, she released her 5th one on her 10th anni too. Only with her, she spaced hers out throughout the 10 years whilst Kumi bunched hers in 5.
Now if you'll excuse me I'm gonna go shoot myself for writing a novel on a forum.
EDIT: It's official, Kuu got #1 weekly

I'm so happy for her <3 Kaela really gave her a run for her money though.
source:
http://www.oricon.co.jp/news/confidence/73210/full/