A 1 defence pure believes that when all stats and equipment are accounted for, having less armour and bonuses but higher stats will give their character the best advantage.
Level 20 defence pures are also called "Initiate Pures" or "Mystic Pures" because of the armour 20 defence allows them to wear. A 20 defence pure believes that the added defensive and attack bonuses that 20 defence armour affords them outweighs the stat advantages that 1 defence pures have. A pure that wears Initiate armour, in addition to defence bonuses, receives a Prayer bonus as well. Initiate pures are the least common of all the pures, but you will see many high-powered pures with 20 defence.
Many times a level 20 defence pure is created because so many mistakes occurred during their 1 defence phase, the added levels to 20 defence were worthwhile for them.