ISO has published an official FAQ on ISO/IEC 29500 aka Office Open XML. The FAQ contains official answers from ISO to common questions such as:

Why two standards?

The ICT industry has a long history of developing multiple standards providing similar functionalities. After a period of co-existence, it is basically the market that decides which survives…

What about contradictions?

…but these can be taken care of during the maintenance of the standard.

Via Doug Mahugh.