What's up with the ninth chevron?

[Sa'ar Chasm]

The whole dialling system makes very little sense. Generally, the Stargate series has been pretty good with their science (Red Sky being an exception), but they inherited a whole lot of gibberish from the movie. You don't need six points to define a location in three-dimensional space, you only need three, although that assumes you're using some sort of grid pattern not based on random squiggles. Assigning each gate a unique six-squiggle address is OK as long as you're not pretending it's based on coordinates or constellations (don't get me started on that...).

Speaking of Red Sky, they don't really explain just how they were able to fiddle with the dialling sequence to get a lock. If a planet has a unique Gate address, shouldn't dialling a different address take you to a different planet?