I think in Sayu's case it's a combination of her massively improving albeit only from horrendous->acceptable (seriously, compare her audition days->2005ish to the recent concerts, you can clearly see how much she had improved since) and the fact that they've found a style that works for her and wasted no time in utilizing it. Meanwhile Chinami... still kinda sounds and sings the same way for as long as I can remember.
For what it's worth though, I'm an avid gamer and after 10+years I still suck hard at rhythm and racing games, competitively speaking, so I don't hold their lack of singing abilities (by her own admission Sayu still counts here, however great her improvement has been) against either of them. Some people are just less inclined to some particular skill than others.
However, that shouldn't stop them from improving in other areas and that's basically what my main complaint for Berryz is