2822upd-06 Sender

Hi, on the DKIM list a poster claimed that Sender is OPTIONAL.
Looking for the evidence why that's wrong in 2822upd I found
an unclear SHOULD:

1:  from multiple authors    => MUST (table 3.6 and in 3.6.2)
2:  one author = transmitter => SHOULD NOT (redundant, 3.6.2)
3:  otherwise SHOULD (3.6.2)

I don't get case (3), why is it only a SHOULD ?  For anything
that's not covered by (2: one author = transmitter) I'd expect
a MUST:

- Otherwise, both fields SHOULD appear.
+ Otherwise, both fields MUST appear.

What could be a plausible excuse to ignore the SHOULD in (3) ?

 Frank