This is caused by the feature that was added sometime earlier.  This feature gives all your ships +2 to total moves in your territory.  It's actually +2 to the total when your turn starts in your territory.  The turns needed calculation uses the total moves your ship has at the start of the turn and the number of tiles it will go through to get the number of turns it will take.

When your ship moves into or out of your territory, the number of total moves it has will update, but the turns needed will not.  Note that the number of total moves the ship has will update immediately when it crosses the border.  When you choose the same destination again, the path updates with the correct information based on the number of total moves the ship has at its current location.  If the path doesn't cross your border at any time, the calculation will always be accurate.

It took me a while to realize what was going on.  Now that I do, I no  longer consider the turn estimate to be accurate, and I don't worry about it.  If I really want to know accurate information, I wait until the ship has run out of my borders to cross.

You can check the total moves your ship has in your territory, then move it the last tile still in your territory.  The total moves it has will be the same.  Move the ship one tile to be out of your territory, and the total moves it has will decrease by 2.  If you move the ship back into your territory, its total moves will increase by 2.  Note that the moves it has left will decrease by 1 in each case.  You may see something like Moves 7/15, followed by Moves 6/13, followed by Moves 5/15.