The main tabs on the Super User homepage use two different colours for the borders:

On Stack Overflow, a single border colour is used:

Similarly, on Meta Super User it looks like:

Is the different colour scheme for Super User (and for Server Fault) intentional? If so, why?

For convenience, here's how the current, all #CCCCCC and all #999999 schemes would compare. I think the "all #CCCCCC" version looks more consistent.