Except, in my opinion, captains don't give the order "travel for 2 minutes then turn," they give orders like "turn to port when we reach that headland."
If they don't, in fact, reach the headland as quickly as they expected, they don't suddenly turn ahead of time until they realise that there's a problem. And the break point for realising that is the turn break. Then, you have the option of continuing to complete what you failed to, or doing something else.
Of course the other factor is to do with gameplay. It's much easier for a GM to interpret orders in a strictly regimented way that everyone understands ahead of time than to try to figure out which conditionals to use. IMHO a good set of rules that isn't FTF could be run automatically, by a mail Daemon perhaps (I remember years ago playing Diplomacy by e-mail where there was no human intervention in running it at all), leaving as little room for interpretation by the GM as possible.