I think how it works, based on the way it does for my account is, all characters in a row that haven't gained exp group together.
For example, on my account, I have 1 Scania, 12 Bera, 2 Broa, 1 Windia, 1 Khaini, 2 Bellocan, 2 El Nido, 1 Chaos character. The 1st, 11th and 12th characters on Bera show active exp - so the groups are the Scania character (on its own), then the 2nd~10th Bera characters, then the 2 Broa, 1 Windia, etc. characters, in 3 separate groups. Plus of course the 3 active characters sitting on their own in the rankings.
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