Well, it is possible that the ninth chevron is an address addition taking into account another dimension that Humans are unaware of and that the Ancients were, that allows for pinpointing a moving object. Based on that:
The Destiny gate is constructed differently than all other gates, so it may have the protocol hardcoded to reset the parameter with each dialing out. As for dialing in, maybe the chevron locking from far away is so much slower because of some sort of 'ping' signal that the gate sends to its destination, that's why it needs more power - to broaden the search for a gate calibrated to receive this ping. That's why Destiny's gate is so unique, the ninth chevron was created specifically for this particular gate and the effect can't be reproduced because the chevron 'slot' is already filled with Destiny's data.
Alternatively: Destiny flies in more or less straight line. With that in mind, the ninth chevron might be a way to pinpoint the location on that specific line, which was known to the Ancients when they launched the ship.
Does that make sense?