These sliders are synced using Animation Sync, but instead of being set directly, the output cube colors are actually being set using an OnDataStorageChanged trigger
There's some glitch where the first time you use a slider the colors will be wrong. To test properly, have each client click on each slider once, and then start testing that they should always match (with 1 second delay due to OnDataStorageChanged rate limit)
On Beta, the cube color will usually not match between both clients (except the first time you join the world)