Lightning damage is lucky?

On my skill tree, under lighting mastery, one of the choices states "Lightning Damage from non-critical strikes is lucky". What does that mean? I know critical strikes can be lucky, but I don't understand how flat damage is lucky?

It does the same thing as with crits: your damage is rolled twice and you get the highest of the two. So if you have "7 to 201 lightning damage", the game might first pick 13, then roll again and pick 78, and so you do 78 damage.

Really good with lightning because it tends to have large variance between the minimum and maximum values.

Roger, that makes sense. Thank you for the information.

if you have a very wide range on your damage, as lightning skills typically have, it comes out to 33% more damage, which indeed very good

For non-critical lightning damage lucky.
What if the lightning damage is converted from physical damage? And the lighning damage is converted to cold or chaos?

More directly, which step does the "lucky damage" mechanism work? Does it work in the first step of flat damage roll or after damage conversion?

It works while it is Lightning damage.