It's the reason Hitpoints, defence, and Prayer are those that are at a lower level. It's hard to know which skills are the least crucial, therefore it's the RS gold individual's decision which ones to keep low and which ones to maximize. When making your decision, be aware that the lower your level is, the less enemies to fight and , consequently, the more successful you'll be.
In terms of the most under-reported stat, it's usually Defence that isn't needed for PVP. The higher the target's Defence level, the lesser is your chance of landing an attack that is successful. However, it does not diminish the amount of damage that you can deal. This skill is also responsible for your ability to wear gear. It is impossible to wear a good armour or helmet without meeting the required level of Defence. This is the most important aspect as better equipment will lessen the amount of harm which you are exposed to.
What makes Defence less significant than the capabilities that are responsible for dealing with damage? The reason is that, in addition to defence skill, you have the Hit Points feature, which determines the amount of health you have. If your health points are high it is possible to survive a lot even with the initial level of defence. With a decent level in your main combat stat (either Attack, Magic, or Ranged) then you'll have the ability to kill your buy OSRS gold opponent before they finish you.