[plt-scheme] "implicitly mutable"?

http://docs.plt-scheme.org/guide/define-struct.html
"#:auto-value auto-expr Specifies a value to be used for all automatic
fields in the structure type, where an automatic field is indicated by
the #:auto field option. The constructor procedure does not accept
arguments for automatic fields, and they are implicitly mutable."

but, apparently there's no mutating set procedure created unless i
also mark it as #:mutable, so i don't grok what it means for them to
be implicitly mutable, if i can't mutate them.

thanks.