Device Status vs Folder Status

I am syncthing several folders between 2 devices and all the folders in both locations show status as “Up to date” and usually Device status is 100%. However when i reload the WebUi in most of the cases i found Device Status going back to syncing 0% or other figure far from 100%, and takes a lot to go back to 100% (lots of files being synced) but folders remain in “up to date” status in both ends. (one is master folder)

Is it really syncing? or is it checking for local new files or local modified files?