...Why is there a difference between "Ambient" and "Internal" temperatures? Aren't they exactly the same, in a functional sous vide setup?
Eventually they will match, but do you leave the food in the tank long enough for the temperatures to even out to that very last degree?
One of the cool things about the sous vide process, is that the results aren't super sensitive to the length of time things spend in the bath (bounded on either side by "not cooked" and the other by "ridiculously long time"). I have never measured the internal temperature of anything I've sous-vided, I've just assumed the internals would have reached the ambient temperature by the time I took them out - I've experimented with 24 and 48 hour time periods... it doesn't destroy the food, but it does change the texture over time, perhaps to something you don't like...
I can see, though, that in a restaurant environment you want to know the earliest you can take them out, and also the latest you can leave them for reasons of hygiene as well as food quality considerations.