I don't know how the count down timer works, but here is a guess. It may be totally off base. It is only based on looking at the figures that you provide. When you get an explanation from somebody with experience, discard my guess.
I figure that 50000 grains/ 24GPG gives 2083.3 gallons. That is how many gallons I would expect the controller to pass through before regenerationg if you had no reserve. 80% of 2083.3 is 1667 gallons.
1667 is close to 1680. So I am guessing the 1680 is how many gallons go through before the controller thinks about scheduling a regen. Once it counts down to zero, it would queue a regen to happen at 2 AM. When the count gets down to zero, the faucet icon may start flashing to indicate a queued regen. Guessing again.
On the other hand, maybe you did not look at the number until after it had counted down some, and my guess is based on mistaken info.
Note there is a better way to set the reserve capacity for the 7000SXT. Ditttohead recommends RS=cr and CR=0. This automates the reserve calculation.
There is also what I consider to be a better way to enter a fixed reserve in gallons rather than percent. But go with the automatic choice unless you have a reason. Fixed 20% reserve seems rather wasteful in many cases.