Scorching ray, from the PHB, works the same way. The reason it does this, as far as I can tell, is that it's more powerful than it should be at the level you get it.
The general guideline for energy damage-dealing spells is that they do 1d6/caster level. But a 3rd level wizard can do 4d6 w/ a scorching ray. Every four levels, they get an additional ray, So they're ahead of the 1d6 guideline at 3rd, 7th, and 11th, on par with the guideline at 4th, 8th, and 12th, and behind everywhere else.
Are the spells you mentioned more powerful enough that the awkward scaling is part of the spell balance?