Triskaidekaphobia (https://en.wikipedia.org/wiki/Triskaidekaphobia). The same thing happened to Microsoft Office - Office 2007 was version 12 internally, yet Office 2010 jumped to version 14. On the plus side, it means that the upcoming release (Office 2016) has an internal version number that matches the year.

I guess the question is whether the people at Microsoft who decide these things are themselves superstitious or whether they think their customers are superstitious (to the point that they would avoid a product with an unlucky internal version number).