I think l1 is considered uniquely onerous because "l" and "1" render similarly in many fonts. That is, the complaint is not about x1, x2, x3, etc., it's about t

I think l1 is considered uniquely onerous because "l" and "1" render similarly in many fonts. That is, the complaint is not about x1, x2, x3, etc., it's about that specific combination.