The rules for Siege Engines were changed in Ultimate Combat.
Indirect-Fire Ranged Siege Engines:
Indirect Attack: To fire an indirect-fire ranged siege engine,
the crew leader makes a targeting check against the DC of
the siege engine. This check uses his base attack bonus,
his Intelligence modifier (if not trained in Knowledge
[engineering]) or Knowledge (engineering) skill modifier
(if trained in that skill), any penalty for not being proficient
in the siege engine, and the appropriate modifiers from
Table 3–12. If the check succeeds, the ammunition of the
indirect attack hits the square the siege engine was aimed
at, dealing the indicated damage or effect to any object or
creature within the area of its attack. Creatures may get a
saving throw to limit the effect of the attack; this is typically
based on the type of ammunition used.
If the attack misses the intended square, roll 1d8 to
determine in what direction the shot veers. A roll of 1
indicates the ammunition falls short (toward the siege
engine), with rolls of 2 through 8 counting squares
clockwise around the target square. Roll 1d4 for every range
increment at which the attack was made (1d4 if the target
square is within the engine’s first range increment, 2d4 if
the target square is within the second range increment,
and so on). The total is the number of squares by which the
attack misses, with the direction in which the squares are
counted determined by the d8 roll. The ammunition deals
its damage and any other effects in the square it lands on.
the crew leader makes a targeting check against the DC of
the siege engine. This check uses his base attack bonus,
his Intelligence modifier (if not trained in Knowledge
[engineering]) or Knowledge (engineering) skill modifier
(if trained in that skill), any penalty for not being proficient
in the siege engine, and the appropriate modifiers from
Table 3–12. If the check succeeds, the ammunition of the
indirect attack hits the square the siege engine was aimed
at, dealing the indicated damage or effect to any object or
creature within the area of its attack. Creatures may get a
saving throw to limit the effect of the attack; this is typically
based on the type of ammunition used.
If the attack misses the intended square, roll 1d8 to
determine in what direction the shot veers. A roll of 1
indicates the ammunition falls short (toward the siege
engine), with rolls of 2 through 8 counting squares
clockwise around the target square. Roll 1d4 for every range
increment at which the attack was made (1d4 if the target
square is within the engine’s first range increment, 2d4 if
the target square is within the second range increment,
and so on). The total is the number of squares by which the
attack misses, with the direction in which the squares are
counted determined by the d8 roll. The ammunition deals
its damage and any other effects in the square it lands on.