Jack is not surprised that they mystify people. They mystify him.
"Yes, it's okay. It's fine. Zel's not good with people." This said with great affection. "He's not as bad as he seems the first time you meet him."
As M says, "It's not quite old married couple and not quite paternal, but Zel tends to sound like a tactless seven-year-old and Jack has to find that balance of saying 'this is really not okay' without saying 'this is really not okay.'" You should hear the ongoing "Now can I kill your ex-boyfriend?" negotiations.