A version number is just an arbitrary point in time, in the case of SPDY, with a certain feature set. Just because SPDY/2 doesn't support everything SPDY/3 supports doesn't mean it's unstable.
This is why I like date-based version numbering, such as Ubuntu 13.10 or C++11.
This is why I like date-based version numbering, such as Ubuntu 13.10 or C++11.