Because ATK isn't a percentage. It's basically an exact damage number. If a monster has 0 defense of any kind, then 250 ATK would = 250 damage.
However, multiple kinds of defense get applied to it. VIT DEF lowers the damage by a specific amount (25 VIT DEF = 25 damage reduction). Other types (Armor DEF, for example), reduce the damage by a percentage (25% damage reduction would reduce that 250 by 62.5).
Also, there are abilities that increase your damage by a percentage (turtle general, for example) and by other specific amounts (samurai for example).
In order to calculate the exact percentage increase in damage output of going from a mace or a chain or an axe up to a Mjolnir, you would have to take all of the above into account.