Not sure if this is by design or a bug.
Great questions, thank you for raising them, as I'm sure others will have the same.
The karma credit total link to the overview page as temporary, as the member view of karma credit log is still in development. Once that log is ready to go, we will link to the log so each member can see what moved the total up or down.
Keep in mind, members will only be able to see their own total, and their own log. Changing the position of the total in the header instead of the profile is also something that we are considering in the context of larger discussions around the public / private view of the profile.
Karma credits versus karma points was selected based on the overall move away from emphasizing points. However, I understand your concern that it implies you can do something with them, and will add it to the feedback list. That may be functionality we rollout in the future, as we consider using some of the additional options available with Answer Hub (use credits in order to perform certain activities).