This seems most likely. In my experience, there is always a 24 hour grace period on KLM - regardless of whether the ticket is booked from the US or Europe.

To the OP; it sounds like a case of confusion (which is understandable, given the myriad rules around ticketing, and that you were booking with considerable urgency). It’s a useful tool if you can’t commit to booking but would like to lock in a fare. That way, if, say, other people were to book the remaining tickets in a given fare bucket during the intervening 24 hours, you wouldn’t get stung.

Of course, the same fare may be available 24 hours later regardless and then you’d be down 40 EUR - but that’s the risk you take and it isn’t going to break the bank.

To be clear; exercising this option won’t actually book the flight. You’d have to go back and do that within the specified 24 hour period. So if I’ve understood your post correctly, it sounds like your daughter didn’t actually book the flight to begin with and you could have just let the option lapse. I understand your confusion though, and why you called to cancel a ticket you thought you’d booked.

I’ve only had cause to use it once myself, but it did come in handy.