Source list (ME01) - system is not validating SA and line item number

Hello Experts,

We have strange scenario in source list (ME01) maintenance. Following is the one of the examples:

1. In year 2013, user maintained an entry in source list with plant (ABCD) and material (A12345) combination having validity date as 01/01/2013 to 12/31/2099 having SA number 5512345678 and line item number ‘1520’ & checked ‘fixed source’ and maintained ‘2’ in MRP relevancy. The SA is related to Stock transport (plant to plant).
2. In year 2015, for same material (A12345) and plant (ABCD) combination user changed the validity start date as 01/01/2015 to 12/31/2099 and retaining same SA number 5512345678, erroneously user entered different line item number ‘1550’ (instead of 1520) and system accepted without any errors. In reality SA line item number ‘1550’ has different material code.

Question is how can system accepts when SA & line item number is different. As I understand system always validates the SA number and its line item number for particular material and plant combination in source list maintenance. In our system I checked for user exits and we are not validating anything. Could anyone help in understanding root cause.

Thanks in advance.

Thanks,

Krish