Our core department has released a new version of Opera Presto which contained support for Opera Turbo, more advanced web standards, Gears and several other improvements. We wanted to make this available to our users as soon as possible, and chose to upgrade the core version in Opera Mobile. Updating the browser core is a big step forward for us, and it felt natural to change the version number.
We have to upgrade to a new browser core as often as possible. Several products using a wide range of browser core versions introduces a lot of overhead (several version of core has to be maintained and worked on at the same time), so we try to use the most current stable core version at all times.
Makes sense to us. Now, can we get a copy of 9.7 already? :)