I've never seen it with icons like that, but every time I've tried casting from Chrome it's been cast the Chrome tab (presumably the top one with the logo) and casting the entire desktop (presumably the bottom one with the monitor), tab allowing you to do things like switch to another window and the video keeps going, desktop showing whatever is on your screen
==EDIT== Nope, tried it out and only got both on sites like YouTube. Looks like it's what others have thought and the Chrome logo is regular casting, the TV logo is something like the old DIAL protocol controlling the app on the box directly
One mirrors the screen while the other for the supported apps and or web pages uses traditional casting.
It does it for nvidia shield tv as well
One casts the full tab while one casts the video straight to the device is what I always thought
Mine looks similar, for me one is Chromecast dongle the other is my Nvidia Shield
One is the tab (top) and the other is directly from the device (e.g. The YouTube app). I have a Shield and they both show up for YouTube while only the tab symbol will show up for others that don't have an app/integration.
My Shield does the same thing, wish someone could provide more clarity as well. It appears at first glance that the bottom option is perhaps the YouTube Smart TV integration while the other option uses the Chromecast protocol . That's my best theory for now anyways.