In the description of Brew Potion, it states that no spell of higher than level 3 may be made. What makes the production of potions of Heal and Cure Critical wounds possible? Are curative spells immune to the rule or are they just actually made as elixirs under craft wondrous item?
Just for clarification, where are you seeing potions of heal or Cure Critical available? That might answer a lot. As far as RAW goes, there's nothing that should allow such a potion to be crafted.
Perhaps those aren't potions at all, but elixers ? (made with the create wondrous item feat rather than brew potion) What's the source ?
An alchemsist can make an extract of Cure Critical Wounds (and Heal, as well), but only he can use it. Unless the alchemist also has the Infusion discovery, which allows others to benefit from the alchemists extracts as well.
In 3.0 and 3.5 (and possibly in 3rd party materials for PF) there were feats and prestige classes that developed ability to brew potions of levels 4th+ but as for now, as far as I know, there is no such option in Paizo-only Pathfinder materials.
In our games there are a few (pre-pathfinder) NPCs knocking arround that the rules purge hasnt caught up with, producing higher level potions so that they are sometimes found in treasure hauls or available to be bought :)
It should be noted that there are also several non-corps who have 50% miss chance rather than 50% damage reduction. Etc
Those suck. On the other hand, we occasionally have a PC sorcerer letting fly with a 15d6 no-metamagic Fireball. Rules dissonance is like cartoon gravity: it doesn't affect you until you notice that it should.
It could also be designers' error. There were few places where NPCs had impossible potions.