I just did the same thing, and found this article to explain 
The Creation Catalyst will not remove tertiary stats on an item. Gear that drops, which gained a bonus stat, is considered a tertiary stat and will carry over when used in the Catalyst. However, stats which are innately built into the original item, such as Speed on High Speed Gauntlets or Indestructible on Disintegration-Proof Waistband will be removed.