One was from Celestial Fire, the other from Celestial Thunder.
The main difference was that the latter had been nurtured and blessed by the Wood God Skill, which guaranteed a certain rate of mutation.
Otherwise, it would be like the Bitter Bamboo. A thousand stalks were burned by Celestial Fire, yet only one became a Tier Four Spiritual Bamboo.
If cultivating Thunder Sound Bamboo had such a low success rate, the Fish Dragon Sect could have planted the entire Hornless Dragon Island with Spiritual Bamboo and still not have gathered enough Tier Three bamboo to refine the Thunder Sound Swords.
The Qian Flame Fire Bamboo Sword itself was refined from Tier Four Spiritual Materials, which was why it could successfully advance with the establishment of the Grotto Heaven Prototype.
By that logic, he could do the same, upgrading one of his Tier Three Flying Swords to Tier Four.
