I doubt someone has modified the table MBEW.
The line item (which is blank valuation type) will always contain the total stock and the total value with regards to sum of all valuation types.
I have tested that scenario and my line item is calculating correctly.
You can see the total value for upper line item is sum of other line items. But in your case, it is different.
As of my knowledge, it can be possible if someone will change the line item manually by hard table update.