Elemental summoning- why is this?

Why is it air elementals are only summonable via scroll, but fire/earth are lvl 4 summons, water is lvl 3.

I think it should be universal lvl 4 summons.  (water elemental would need to be buffed slightly)

Overall, there is a need for a few more spells in the elemental schools

2 Spells for each as an idea

Air:

Summon Air Elemental- level 4

Suffocation- lvl 2 air spell, does damage each turn if not resisted,   Additional damage per shard.   16 mana

Earth:

stoning: lvl 1 earth- does minor physical damage, but dodgable.  Additional damage and accuracy per shard   8 mana

crystal shield lvl 4 earth- allows for absorption of 1 spell, and the mana added to the caster's mana pool.  Does not work on area spells  50 mana

Fire:

Immolation: lvl 3 fire spell +1 damage and -1 defense per fire shard  15 mana

Volcanic Conduit: lvl 4 fire spell 2 turn charge time.  does damage per turn if not dodged.  Dodging ends spell  Shards increase accuracy.   30 mana+5 per turn

Water:

Frostbite: lvl 1 victim takes 1 damage per shard per weapon swing.  10 mana

Great Hail: lvl 3:  hits an area similar to fireball 1 turn charge time  10 attacks in a random square+5 per water shard.  20 mana