the mastery tree gives 5% of 5735 wich is 6022 mana might give some awesome dmg xD.
and you are still tanky. and those 6k is without the R spell wich gives 225 in lvl 16, and that gives a total of 6247 mana 62 is 1%

Why 0/9/21 instead of 9/0/21 ?

because the strength of spirit mastery gives 1% of your mana in hp reg/5 and just the masteries gives you 60 hp reg/5, when you reach that 6k mana

Arcane Mastery: When Ryze casts a spell, all other spells have their cooldown reduced by 1 second.

Overload: Ryze blasts an enemy unit for 30 / 60 / 90 / 120 / 150 (+20% of ability power) plus 8% of his max mana in magic damage. 8% of 6k is 480 ouch

Rune Prison: Ryze traps an enemy unit, preventing them from moving for 1 / 1.25 / 1.5 / 1.75 / 2 seconds and dealing 60 / 95 / 130 / 165 / 200 (+60% of ability power) magic damage plus 5% of Ryze's maximum mana. 5% of 6k is 300!

Spell flux: Ryze unleashes a bouncing orb of magical power which bounces to enemy units or himself, up to 5 times. Each bounce deals 50 / 70 / 90 / 110 / 130 (+35% of ability power) magic damage and reduces the target's Magic Resistance by 15.

Desperate power: (Passive): Ryze gains 75 / 150 / 225 bonus mana.

(Active): Ryze becomes supercharged, gaining 15% / 20% / 25% spell vamp and causing his spells to deal 50% AoE damage.