It seems to be normal for Macs that have an optical drive now, due to Apple's strange decision to relocate Startup Disk from the bottom row, where it had been for years. Sierra compared to El Capitan:
View attachment 663318View attachment 663317
Newer Macs without an optical drive/icon for CDs & DVDs don't have this issue, and the icons fit in a neat row on Sierra as well. Possible to fix by going into Customize mode and hiding this icon.