Does carbonated water change over the course of several days, or was my container leaky?
The difference is in what is leaking. Gas leaks out a lot easier than fluid. The container is rated not to leak liquid, not that it won't leak air. So if you turned it upside down with 35 psi inside (a carbonated drink) the fluid won't leak out. 

So turn the Klean Kanteen flip-top growler upside down. ;-)