I always run emerge -pvuD world to check updates, then sudo emerge -quD world to update my Gentoo. Sometimes, I would see packages showing up but I couldnt understand why they were pulled for update. Here is a case:

https://farm5.staticflickr.com/4138/4886199841_fdd3a34807_o.png

chromium package is no doubt that it needs to be updated since its in world (rendered in bold style). But for eventlog, you would not think its a dependency of Chromium; and for librsvg, its not one because itd be updated after Chromium.

So, why do they show up? After reading the man page, --tree (or -t) option would tell you the reason.

https://farm5.staticflickr.com/4117/4886199913_e2da2e382c_o.png

I can see clearly now.