How is attack strength calculated?

Sometimes I just can't figure out how the unit strength value is calculated in combat. For instance, yesterday I was sinking poor outdated barbarian ships with my sub, and when peeking at the combat preview I saw this:

Why is the caravel's strength listed as 6 and not 15? And why is the sub's strength listed as 130.8 and not 60 * (1 + 0.6 + 0.33) = 115.8? Does anyone know the formula the game actually uses?

130.8/60 = 218% =

• 100% (base)
• 33% bonus vs barbs
• 60% bonus vs domain (that's targetting 3, right?)
• 25% unlisted bonus (it's unlisted because the screen is too short to list it)

A few possible sources for that 25%...

• Himeji Castle for 15% in friendly territory.
• Nationalism Social Policy for 25% in friendly territory. <-- we have a winner.

As far as I know, all combat modifiers are additive.

Ranged units use ranged strength to defend ranged attacks. Thus I can only assume that there's a hidden -15% modifier to the barbarian's 7, reducing it to 6 str.

It appears that the sub is being affected by an unlisted/hidden or off-the-screen 25% bonus that you aren't seeing:

60 * (1 + 0.6 + 0.33 + 0.25) = 130.8.