I think what he means more is that Annette has had the tendency to be off. Key, tone, whatever. Tarja I don't think I've ever heard her be off. You may not like her extremely operatic style and prefer Annette's more rock/pop vocals, but you can't say Tarja's off.
I have a classical instrumental background as well as some rock experience on bass guitar and background vox and I have no trouble with operatic styles - I like Simone Simons' (Epica) vocals quite a bit - but for whatever reason Tarja's voice just grates on my nerves to a degree.
One thing I can say for sure is that no matter how easy it seems to perform - instrumentally or vocally - in practice, or in a studio, or in some other controlled environment - that all goes out the window when you're on stage in front of hundreds or thousands of people. Many people cannot handle this kind of pressure, and for most others it leads to increased nervousness either consistently or on occasion, which can tighten up the vocal cords and raise your voice by between a quarter and a half step. Which is disastrous. It's almost impossible for singers to overcome this kind of nervousness on stage because if you try to force everything back in tune, you risk making it even worse. This appears to occasionally be Anette's problem...but again, there are so many bad rock singers out there that butcher almost every show they do that her having the occasional bad performance shouldn't really register as too much of a blip on the radar.
Some vocal types are more suited to live performances - Tarja seems to have one of those voices based on available youtube clips. Voices with stronger vibratos like Tarja's and Simone's are better able to mask any off-key notes because it's easier to pull a voice that has a constantly changing sound back into tune without it being noticed. Anette's voice is more of a contemporary sound - very little vibrato in it and very smooth and steady - and therefore, because of her vocal TYPE, it's harder to keep it in tune or to pull it back into tune without it being noticed if you screw up. This has nothing to do with their capabilities as singers and everything to do with how they were trained and what type of voice they have (which goes back to training and individual preferences of each singer).
Not to mention any external factors not within the singer's control. Sickness? Vocal fatigue? Physical exhaustion? Pregnancy?
Additionally, every stage and auditorium/stadium/arena has different acoustic properties. A few singers can detect the subtle differences and compensate accordingly - usually those who have been trained since they were children. Most singers, however, can't do that.
This doesn't make either a bad singer. Heck, how many singers sound good on the record and then butcher every single goshdarned concert they play? Seems like well over half have this problem. It's incredibly rare for a vocalist to go two months - nevermind a year - without two or three off nights where it's just not there somehow. In opera, you have an understudy or two to step in if it's not there in warmups. Rock bands don't have that fallback.
No vocalist or musician is perfect, and it's unreasonable to expect a vocalist or musician of ANY kind to be perfect every single performance they do. That is what some people here seem to expect out of Anette, and THAT is unreasonable and unattainable.
