Wow, that's weird. It could be your amount of trade, actually, though I'm not sure why it would be under "Diplomacy" if so. That the Drath and Terrans (who have diplomacy bonuses) are on top suggests that it is, however, related to Diplomacy. It could also be espionage, or (more likely) it could be your popularity and your popularity could be something completely different (like how many enemy planets were affected by your influence zone).