Requsition gets approved beyond the approval limits of the preparer

Hello All,
One of the employee has the approval limit of 2500 Eur, the Purchase Requisition worth 4500 Eur got approved,with out going to the supervisor for approval.

I have analyzed the below points,

1. I have check the approval group and the assignment for the employee job, it is setup to approval limit 2500 eur.

2. The employee has the supervisor assigned,the approval limit of supervisor is higher i.e to 7500 eur.

Please suggest how i need to trace this issue.

Thanks in advance.
Deepak