Infernal Healing and Celestial Healing are similar spells and seem to be opposites of each other, but the former has a duration of 1 minute while the latter has a duration of 1 round/2 levels (which is much worse). Is there any official explanation for this discrepancy?
I'm not aware of an official statement from the developer that wrote the spell regarding the discrepancy.
There are statements around Infernal and Celestial Healing that we can use to infer the dev reasoning though...
Per creative director James Jacobs in a 2011 thread on Infernal Healing on the Paizo.com forums:
[...]we won't be printing a "celestial healing" spell ever.
Infernal healing is one of many ways that we try to present "evil" as being sort of seductive. It's not intended to replace spells like cure light wounds. It's intended to be something that a devil lets you do but at the cost of a little bit of your soul.
Good creatures don't work that way. They'd just give you a fast healing spell, or a cure light wounds, or whatever.
This is another case where symmetrical design isn't necessarily good for the flavor of the game.
I would remove it from the game.
Infernal healing is as much about temptation to utilize evil as it is about providing healing for a category of creatures that have more difficulty getting magic healing than most others (since evil clerics can't generally channel to heal). Furthermore, I'm a big fan of asymmetry in design. Infernal healing is more interesting if there's NOT an "opposite" spell.
A spell that has a duration of 1 round/2 levels should have a minimum duration of 1 round, in other news.
(Caveat as always is that JJ is a creative director and not a rules guy)
While JJ is not a be all end all source, you can see that there is at least an awareness among the devs that Infernal Healing is inherently about temptation. Its the "best"/most efficient low level healing spell, but it carries the evil subtype and will turn your alignment to evil if you use it too much.
Without that trade-off, Celestial Healing just couldn't be as good.