This is the usual completely meaningless accounting, with a myriad of methodological flaws. You cannot make a general statement about bugginess of open-source vs. closed-source code. There are just too many variables to conduct a statistically meaningful study.
The reason why open source code is better bug-wise is very simple: the user of the code can fix the bugs. As a developer, I hate to depend on closed-source code. Why? Because *every* moderately large code, open or closed, has bugs, and they invariably show up at some point. I much rather go fix the issue myself that wait for some random amount of time (including forever) for someone else to fix the bug for me.