Those others field types have a standard or system-dependent fixed size and a number is an optional repeat count. Format 's' is always a single field because number prefix for that is special -- a field size.

"For the 's' format character, the count is interpreted as the length of the bytes, not a repeat count like for the other format characters; for example, '10s' means a single 10-byte string, while '10c' means 10 characters. "

I happen to think is could use a few more words of special explanation. Looking again, I think ", which defaults to 1" after the first 'count' above, would be sufficient. We nearly always docucument defaults.