That has to do with the turn order. If an enemy enters/passes your territory before your force has left, they encounter eachother, and the same goes for you when you move. When you are first, or when no enemy moves before you, you know exactly where the pieces will be during your move. Conversely, when I go last I often command my units to act attack (leave their defended countries) because they will not leave if their country is attacked.
The order changes every turn. (I think you go 4th, 3rd, 2nd, 1st and then 4th again. I also think this is mentioned in the manual or some other documentation.)
About the confirmation button: Do you know that there is an "undo" feature? You can press backspace or use the little button (turning arrow near the research buttons, I think). Undo should be a better solution, because it doesn't add extra steps when you do the right thing, and functionally it is simply more powerful than prompting to confirm. (You can cancel one operation, same as with confirm, or you can cancel several.)
Confirm/cancel and undo aren't mutually exclusive features, but confirm isn't strictly necessary where undo can be employed.
