If they think they should be rated higher, then they should give the reasons why.
If those reasons are valid, then the employer can change the rating. If the employer is unwilling to change it for irrational reasons, that means that the rating system is not objective and the system is undermined and has lost its meaning. This becomes a separate problem entirely.
If the reasons for valuing themselves more are not valid or applicable to the company, then the employee is over valuing themselves in this context. If the employee is not willing to accept this, then they have the option of declining the offer and moving on.
If the reasons for valuing themselves more are not valid or applicable to the company, then the employee is over valuing themselves in this context. If the employee is not willing to accept this, then they have the option of declining the offer and moving on.