When you have a repeating command block with this command:

/execute as @e[type=arrow] run data merge entity @s {NoGravity:1}

and you shoot an arrow with a low velocity. Repeat a few times and watch as they start to fall and readjust themselves with where they actually are, you can also use a particle command inside an execute command targeting the arrow and you can see where the server thinks it is and where there is a desync with the client.
This effect can be better seen when the arrow was shot into a block and the block gets removed afterwards.