It's not only restart() and reset(); the member functions start() and stop() would have to return the elapsed time for consistency, too. In contrast to sf::Clock, the classes thor::StopWatch, thor::Timer and thor::CallbackTimer have many attributes (not just the time), so it is not immediately clear what the return type represents. Especially for timers, it may not be obvious that the returned value would be the remaining and not the elapsed time. That's why I prefer the explicit getters -- in most classes, modifiers and getters are separated anyway.
This might change in the future, but by keeping void for the moment, I'm not going to break any code, in case I should once find a more appropriate return type than the elapsed time.