Only one! That's so few as to almost be suspicious.

There's only so many names it makes sense to use for, say, a filename variable.
It's trivially easy to globally change variable names anyway, so it's not really an indicator of anything. Seeing how closely code logic in one product follows the other would be the only way to tell if code was stolen.
Personally, I'm a fan of open-source anyway. Programming, at least in my field (web applications) is more like providing a service than generating new intellectual property. The more code is shared, the better we can do our jobs, the better results our clients get, and a reservoir of free code gets developed that can be used for non-profit uses. That's all good.