Does anyone happen to know why Sven has a rank of -1 in the bard singers faction while all others have a rank of 0?

maybe he got kicked out?

I wish he would. I'm trying to make a mod that stops them from singing, but doesn't stop them from playing instruments. Moving everyone from the BardSingerFaction to the BardSingerInstrumentalOnlyFaction should work, but before I do this I need to know why Sven is different (i.e. the only one in BardSingerFaction who has a -1).

While any character you create has -2 rank in any faction you didn't add to and removeing a NPC from a faction will give also a -2 rank, any negative rank means that it's not member.

Beth use the -1 as a convention for characters they plan to add to a faction. That could mean that he is added to that faction in one of the vanilla quests, or maybe that he was supposed to be added but finally they decide not to do it.

It could be because unlike all other bards, he's available as a follower--if he were fully in the bard faction, the bard AI packages might conflict with the follower packages. Just a guess, though.