First off, don't forget that the previous attack spells were split in to
The first, the "arc" type spells, have TN 16 to make what amounts to a melee attack. For an increase of 2 tot he TN that becomes a bolt spells.
The sphere/ball spells all explode into a 5' radius of the point of attack (i.e. a 10' diameter sphere), so having a higher TN is appropriate since it can attack more than one foe at a time, at range.
And if you look at the casting options, +2 TN to take adjacent attack and make it 50' range. And under the sphere spells, we are talking another +2 TN to increase radius 5'.
so, thus CTN of 20 (4 points higher than the arc spell).