I'm not seeing a point behind the pedantry.
Let's compare shall we?
5th level Ranger attack spell - Conjure Volley: Range:150 feet, area 40 foot radius damage 8d8 (type varies by material component used)
5th level Wizard attack spell - Cloudkill (just because I hate flipping pages): Range:120 feet, 20 foot radius, 5d8 poison damage (Concentration up to 10 minutes).
Looks pretty darn close to me. Cloudkill might be a bit more effective, simply because of duration, but, then again, it's half the size. Used for maximum effect (ie, on as many creatures as possible), cloudkill would take several rounds to catch up to Conjure Volley to do the same total damage.
Cone of Cold - also a 5th level wizard spell deals 8d8 in a 60 foot cone. Again, same as Conjure Volley.
Seems pretty on par for spells of the same level.
Conjure Barrage - 60 foot cone, 3d8 damage. Significantly less than a fireball, but, you can also vary damage type, making it effective against fire resistant creatures. I'd still give the edge to fireball at 8d6, but, then again, we're not talking huge differences here. 3d8 averages 14 damage, 8d6 averages 24. Extra 10 damage on average, so, it's not like it's overwhelmingly better.
By and large, spells follow the chart on page 284 of the DMG - Spell damage. It doesn't differentiate those spells by class or by when a given class gains access to that spell. Damage is determined by spell level, period. A 5th level multi-target spell should do 8d6 damage. Conjure Volley is actually a bit strong for a 5th level spell. OTOH, Fireball is a bit strong for a 3rd level spell.
The question becomes, why? Why are these particular spells a bit stronger? Well, in fireball's case, everything and it's mother gets fire resistance. A LOT of creatures are resistant to fire, so, fireball, while a bit stronger over all, has a pretty big blind side. Same with Conjure Volley - these are non-magical ammunition - anything with resistance to normal damage is taking a lot less damage. Again, a fairly common resistance.
The power of a spell or a mystic power is not relative to the level at which you gain that spell. It is relative to the level of other spells/powers of the same level.