Pitch of third person player model isn't predicted
When using cl_movement 1 (client-side prediction), which is enabled by default, only yaw is predicted. If your pitch changes, you will see your player model lagging behind.
As a side note, perhaps a constant client-side “lag” of eg. 50 ms could be added to both pitch and yaw to make third person playing look and feel better. This is already done by games like Red Eclipse.