This is another one of those issues that has been around for a long time and, since it is still present in the 1.10 release, I want to bring it up again to make sure it is on your list of things for the devs to fix.
The issue is that the game "miscalculates" the number of turns it will take to move a ship/fleet from its current position to a target destination. It does not do this every time, but I estimate (rough guess) that it does this about 1 out of 5 times.
The first screen snapshot below shows a fleet of 12 ships that I want to move to a distant destination. It shows that it will take the fleet just over 3 turns to arrive at the target cell. The second screen snapshot shows that the fleet can move 13 cells per turn but, if you look carefully at the first screen snapshot, the projected path shows that the fleet is expected to move 15 cells per turn, not 13.
Now, if I right-click on the cell near the end of the projected path that has the number "3" on it (indicating that the fleet should be there after 3 more turns) the game recalculates the trip correctly as shown in the third screen snapshot below.
This is not a show-stopper, end-of-the-world bug, but it is very annoying and should be easy to fix.