Are you giving the "synchronise arrival" order to all units at the same time? Ie. you select the units from, say, all three adjacent provinces, and THEN opt to attack and synchronise arrival?
Or are you using the order on a province, or even unit, basis?
Another one that I *suspect* exists, but isn't too common that I'm sure, is that terrain traits affect movement speed, but synchronisation is NOT updated, so a general that gets the trait during the battle that should result in synchronous arrival, will arrive sooner than others.
If it's neither of those, I'm at a loss.
Or are you using the order on a province, or even unit, basis?
Another one that I *suspect* exists, but isn't too common that I'm sure, is that terrain traits affect movement speed, but synchronisation is NOT updated, so a general that gets the trait during the battle that should result in synchronous arrival, will arrive sooner than others.
If it's neither of those, I'm at a loss.