Starting with MATLAB 7.0 (R2006a), MathWorks has been releasing twice a year ever since then -- hence the "a" and "b" releases.
Before then, MathWorks didn't use the year. So, R14 (and its service packs) were the last releases before switching to the current R<year>a + R<year>b notation.
You could always check out the Wikipedia entry on this: http://en.wikipedia.org/wiki/MATLAB#Release_history
- Sebastian
