I noticed the same thing as OP... this is what happened in 5.0, when 5.0.1 was out, settings showed a notification badge as soon as you checked for the update. So if you tried checking for update and didnt actually download the update, the notification badge was still seen.
When 5.1 update came out, I checked for the update but didnt download the update, the notification badge on the settings icon didnt show up. After I downloaded the update, but didnt update(because battery was too low), the notification badge was shown once again.
I guess thats one of the changes in the settings notification badge display or a bug, who knows...