The ones I was referring to only have 2.4x speed supported. They have been out on the market longest - so in most cases newer writers have support for those discs. The 2.4x - 6x media is newer and not all writers support those.
Based on this:
I do get a feeling that the writers firmware don't include full support for those discs. If it had support for them - the 'Speeds:' info should have included this (Speeds: 2.4x, 4x, 6x). And as it can't fully support those discs - it can't burn any slower or faster than 4x on that media.