Since the [website][1] reads `Weapon: +12 Minimum Damage, +12 Maximum Damage`, I suspect it is just an incorrect tooltip

Its likely that at one point during testing the gems increased the maximum damage by twice the minimum, however this probably got scaled back and the tooltip never updated

**EDIT:** This actually isn't correct.

Rubies provide a flat increase to weapons, however how much that increase is is based on weapon speed. See [this answer](http://gaming.stackexchange.com/a/69265/8462) for more details

  [1]: http://eu.battle.net/d3/en/artisan/jeweler/recipe/radiant-ruby