Following the same priciples as the CD-R move from 74 min to 80 min, tightening the spacing.
It seems it is not certain if it remains within the tolerance limits (like 80 min CD-R did) or violates them (like CD-R 90/99).
Compatibility with drives other than those indicated may be uncertain, for one thing, I cannot recall a single drive ever managing a "DVD overburn" - even the ones mentioned, so I guess this must report the actual capacity.
200 MB extra capacity seems a small reward for the risk, since it is also likely to have an adverse effect on quality.
If double layer wasn't available, then maybe there would be more point in pushing the limits like this, but it does seem to be too little, too late.