It is mentioned several times that mirror-urls (SYNC, GENTOO_MIRRORS) should not end with a trailing "/". In the manpage of make.conf PORTAGE_BINHOST is explained with, as example, a (fictive) URL with a trailing "/". I don't know if this is an error in the documentation, or if PORTAGE_BINHOST really should end with a trailing "/". It might be wise to use the same convention for PORTAGE_BINHOST as for GENTOO_MIRRORS and SYNC.
it doesnt matter whether you have a / on PORTAGE_BINHOST but i'll remove the trailing / from the man page