These deviations are likely to compensate the Archon's inability to spontaneously cast cure spells
Monsters do not have to strictly follow the same rules that apply to PCs (Bestiary, p. 290):
Creating a monster is part science and part art. While most monsters follow a general pattern of their overall power and abilities as related to their Challenge Rating (CR), there are many exceptions.
In particular if you look at spellcasting:
Spells Known/Prepared: If the creature can actually cast spells, its caster level is indicated here followed by the spells it knows or typically has prepared. Unless otherwise indicated, a spellcasting creature does not receive any of a spellcasting class’s other abilities, such as a cleric’s ability to spontaneously convert prepared spells to cure or inflict spells.
If you look at the deviations for the Archon, the total number of spell levels they have is very close overall to what the normal number would be: they gain +1 on first level, +4 on second, and lose 4 on fourth, for a total deviation of just one spell level overall.
Then, if you look at their spell selection, you can see for the first 2 levels
1st—bless, cure light wounds (3), divine favor, sanctuary (DC 16), shield of faith
2nd—bull’s strength, consecrate, cure moderate wounds (2), lesser restoration (2), owl’s wisdom
It’s pretty evident that these extra spells help compensate the Archon's inability to spontaneously convert other spells into cure spells. Otherwise you'd have to sacrifice either the variety of different effects available, or the ability to cure wounds. It is a classical trope for angels to heal, restore and strengthen their god's believers, and this is expressed in these spells. An Archon who only had a single cure spell for the day would be an underwhelming show in that respect.
To compensate for the extra spell levels to some extend, the Archon has one less higher level spell. The designers thus deviated from the rules that would apply to a PC cleric that has such spontaneous conversion.
I don't have any designer commentary supporting this is what happened here, making this a reasoned conjecture, not a proof, and I don't know why the did not trade a level 5 slot instead to exactly balance it out. Maybe they felt a level 4 was worth more than 2 level 2 due to action economy considerations.
You mentioned in commentary that your concern is how many spell slots the Archon has RAW, the number in the monster stat block, or the number indicated by Wisdom and Cleric level 14. Following the Specific beats General principle, RAW the Archon has the spells and slots that are given in the monster stat block, no matter how the designers arrived at them.
