Every person who worked on WordPress has wondered some or the other time that , Why they change the default theme every year. So it all started in 2005 when Kubrick launched a default theme, but didn’t change it for coming 5 years.
With Twenty Ten, started a new trend where a new default theme is introduced for each year by the name ‘Twenty ___ ” representing the year in it. This gives theme a lifetime and an expiry date so that new features can be added at regular time intervals.
The concept of Default theme was based on idea that a single theme cannot do everything but can still be simpler while fulfilling all the principles WordPress is built upon. Default theme will give an idea of all the latest and cool features but yet be simple, while aligning to the principle of Globalization.
Ofcourse the new default theme will have a different layout from the previous one, but the best in coding principles are always kept into the design frame.