When the world is not enough?

[thescorpionking]

What would cause my /var/edb/world file to get out of sync?

For example,

emerge -pud world
[ebuild U ] sys-libs/db-3.2.9-r5 [3.2.9-r2]
[ebuild U ] dev-libs/glib-2.2.1-r1 [2.2.1]
[ebuild U ] net-im/gaim-0.63-r1 [0.63]
[ebuild U ] app-misc/muttprint-0.71 [0.64]
[ebuild U ] net-mail/mutt-1.5.4-r1 [1.5.4]
[ebuild U ] sys-apps/hotplug-20030501-r1 [20020401-r1]
[ebuild U ] net-print/cups-1.1.18-r5 [1.1.18-r4]
[ebuild U ] app-editors/vim-core-6.2_pre6-r1 [6.2_pre6]
[ebuild U ] app-editors/vim-6.2_pre6-r1 [6.2_pre6]

yet emerge -pud mplayer

[ebuild U ] dev-libs/glib-2.2.1-r1 [2.2.1]
[ebuild N ] media-libs/libsndfile-1.0.5
[ebuild N ] media-libs/faad2-1.1
[ebuild U ] media-sound/esound-0.2.29-r1 [0.2.29]
[ebuild U ] media-video/mplayer-0.90-r1 [0.90]

What would cause mplayer not to be in my world file?

Why doesn't regenworld fix this problem?

Thanks.