sys-process/psmisc is missing a slot dependency on sys-libs/ncurses, so since the creation of the slot for binary packages that need libncurses.so.5, portage will attempt to pull in that slot, since it's got the best visible version in it. It should probably also have a slot operator dependency, because pstree links against libncurses.
commit 31a8766e174a9063a21d0c359750f9cbafbb8e0d Author: Lars Wendler <polynomial-c@gentoo.org> Date: Thu Oct 27 08:02:28 2016 sys-process/psmisc: Revbump to add sub-slot dependency for sys-libs/ncurses. This was requested in Gentoo bug #598166. Committed straight to stable where -r2 was stable as nothing changed except for the EAPI and the ncurses sub-slot dep. Package-Manager: portage-2.3.2 Signed-off-by: Lars Wendler <polynomial-c@gentoo.org>