How does discovering new patterns/recipes/schematics with reverse engineering work?

So, the short version is, You Got Lucky.

Here's the long version:

There are two kinds of reverse engineerable items. The first kind is simple. Reverse Engineer the item, learn the pattern for a slightly better version with a higher color-grade quality, up to Artifact grade (Purple). The chance to learn the higher-quality schematic is usually 20%, you can see the chance when hovering over the item. Most crafted item customizations (Armoring, Barrels, Mods, Enhancements, Crystals, Hilts) and consumables follow this system, as do some craftable weapons (but not most). A crit on crafting one of these results in two of the item rather than one.

The other system is a bit more complicated. Most equippable items, when Reverse Engineered, have a chance to teach you a variant of the item, which is upgraded. The new recipes name will have one of three prefixes, based on the additional stat that is added to the item:

• Redoubt _______________ items will have increased Defense Rating.
• Critical ___________________ items will have increased Critical Rating.
• Overkill __________________ items will have increased Power.

Unlike critical crafting with base items, a crit on one of these Prototype items will instead be called Advanced when they gain an augment slot.

These prototype items, like customizations, can be RE'd again, to give you a chance at an Artifact grade item. Once again, there are a number of variants - specifically, for each prefix, there are five variants that can be learned. These variant items will have new prefixes, and add two additional stats to the base item. These items will also have the suffix [Superior] unless crafted critically (i.e. with an Augment slot), at which point the suffix is changed to [Masterwork].

Overkill items can be turned into:

• Vehemence, which increases Power and Alacrity.
• Rampart, which increases Power and Shield Rating.
• Commander's, which increases Power and Presence.
• Expert, which increases Power and Surge.
• Hawkeye, which increases Power and Accuracy.

Critical items can be turned into:

• Leadership, which increases Critical Rating and Presence.
• Tempest, which increases Critical Rating and Shield Rating.
• Fervor, which increases Critical Rating and Accuracy.
• Endowment, which increases Critical Rating and Surge.
• Supremacy, which increases Critical Rating and Alacrity.

Redoubt items are a bit of a special case. While 4 variants are known, to date, nobody seems to have been able to craft a +Defense, +Alacrity item. Whether this is a bug (people have invested a lot of time into researching this), or simply cosmic levels of bad luck is at this date unknown. That said;

Redoubt items can be turned into:

• General's, which offers Defense Rating and Presence.
• Veracity which increases Defense Rating and Shield Rating.
• Anti-Armor items have added Defense and Surge.
• Exactitude will feature improved Defense and Accuracy.

The image below provides an example of all the variants that can be crafted of one item:

(H/T to the incredible research done by Slaign over on the official forums on this one.)