It's to do with the dispersion pattern of (mainly) the tweeters. You will have seen on another thread about the small "sweet spot" reported with Sonus Fabers. This is because of the directional characteristics of these units.

Meridian speakers (and my ATCs) are just fine with the centre a bit lower than the main L+R pair. Mine are very well integrated so ,as Nick says, it is pretty much perfect.

Malcolm