When a score that is displayed on the side bar changes, it is not reflected on the side bar.
Steps to reproduce:
Create a new world with commandblockoutput set to false (as this updates the UI with chat)
Do this command: /scoreboard objectives add test dummy
And this one as well:/scoreboard objectives setdisplay sidebar test
get a command block and set it up to repeat its self with this command:
/scoreboard player add @a test 1
Notice how when you touch a different item on the hot bar, the number suddenly jumps to the current score value.
The displayed value would update every time it changes (like how the coordinates update every time they change)