Entities attached to a player-controlled unit through the "attachTo" command lag behind the attachment point in their position and rotation while the unit is in a vehicle and in motion. This is not exclusively a network/multiplayer issue because it exists both in singleplayer and in multiplayer on local-only objects, neither of the two cases would involve networking of position or rotation.

Replication:
1. Spawn some entity to attach to the player (anything works, but small enough to notice the difference, Sign_F will do).
2. Attach the entity to the player, like with: "entity attachTo [player, [0, 0, 0], "spine3", true]".
3. Enter a vehicle and drive around. A kart is ideal because it is fast and the offset scales with speed.
4. Notice the entity gaining a constant offset behind the movement vector and sometimes jittering to an even further offset on some frames.