Since the website 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 some weapons, for others however, there is a range that is based on weapon speed. See this answer for more details
