How is embarked movement increased?

What are the ways that embarked movement is increased with Civilization V? I am playing with the Brave New World expansion.

What I would expect based on the tech tree and Civpedia is that Optics gives embarkment with 2 movement points, Astronomy increases to 3 movement points, and Steam Power increases to 4.

When I was playing with Austria recently, my embarked units have 4 moves, but I had not researched Steam Power. When I researched Steam Power, my embarked units had 5 moves.

I feel like I am missing something.

I did some searching for "Civilization V embarked movement", but I only found information that was either not updated for BNW, wasn't specific enough to explain what I saw, or wasn't any different than what the tech tree and Civpedia said.

I am playing with G&K and BNW, but no mods.

Update: The units did not have the mobility promotion, nor any promotion that would explain the extra movement.

Update 2: I have been unable to reproduce the unexpected embarked movement. Perhaps it was a rare bug?

Technologies:
- Optics +2
- Astronomy +1
- Steam power +1

Policies: Exploration +1

Unique Traits
England +2
Denmark +1

Ideology:
Lightning Warfare: Armored units receive +1

Maybe you had exploration?