When casting spells that are Vs. DEF, there is only 1 roll that is made.

First, the player makes his casting roll, adding in his Spellcraft skill bonus and any other modifiers. This is then compared against the Casting Target Number (CTN) for the spell. If it is equal to or greater than the CTN, then the spell was cast correctly.

The next step is to compare this SAME result against the target’s DEF. If it is equal to or higher than that, then the attack hits. The damage, including Scalable Damage, is then resolved just as with any other attack.

It is quite possible to successfully cast the spell and still miss the target.