There are many other factors affecting the outcome of a land battle. Supplies, armor, number of guns, fortification, combat stance, disruption level, fatigue level, command infrastructure bonuses, unit cohesiveness, etc.
Yes, all of which apply EQUALLY to both sides so they are not determining factors unless playing against a newbie.
Supplies: Eliminated by bombardment/air attack. Brought by transports for attackers. I have NEVER launched an attack that wasn't over long before supplies ran out.
Armor: None in the game during the time period PM will fall. Non-Factor
Guns: Both sides have access to these assets, Japan generally has more being that THEY are attacking and have to bring 2:1 worth of ODDS. Northern Command CD units can not be relocated to PM.
Fortification: Quickly eliminated by (a) enough troops to supply 2:1 odds and (b) combat engineers. Since PM is generally only going to be a 4, it vanishes almost instantly.
Combat stance: Attacker only posture. (there is no Shock Defend option)
Disruption/Fatigue: Quickly dealt with by ground attack and equalizing quickly after a round or two of ground combat. Proper odds results in low disruption rates on attacker side. Misuse of Shock attack seems to be the culprit here.
HQ Bonus: applies equally to both sides.