Except after applying rule 1 here, you have 6 (div) 2 3 where there is an implied "(mult)" between 2 and 3 so you really have 6 (div) 2 (mult) 3 and your rule 3 does not specify which comes first. If mult comes first, you have 1, if div comes first you have 9.

Mathematics requires the removal of ambiguity; your rules did not achieve that.

Another way to look at it (the way you actually look at it when you do mathematics) is, you need one more pair of parentheses here to write it properly, so that it is not ambiguous.