If this is the case, then I believe that the rules should be reworded, since Target Number paragraph explicitly says that artillery attacks (except artillery cannons) don't use range modifiers, while the Range paragraph just as explicitly calls the +6 mod a range modifier.
Edit: I need to apologize to everyone reading this thread, especially nckestrel. It turns out that I was working with an outdated Fifth Printing pdf when I wrote all of the above instead of the current Sixth Printing, which has some changes on p. 47. In particular id does not mention a +6 modifier for artillery attacks at all. Still, if what nckestrel said above is correct (and the footnote 12 in the Attacks Modifiers Table on p. 44 supports him) I believe my original response to him still stands for the most part - page 47 needs another correction, because right now it tells to use a flat +4 modifier for most artillery attacks at all ranges, including extreme.
Once again, sorry for causing unnecessary confusion.