You book the first and only Goods Receipt for a vendor and later you make a reversal of it, system doesn't take back the scores calculated for this goods receipt.
The score calculated during a vendor evaluation is an average of all documents post in a period for this vendor.
When a score is calculated for price, for instance, system defines the price score for each order and then smooth it between them, but at the end system doesn't know how many orders composed the final score, only knows the score.
When you reverse a goods receipt, system define the score and then try to withdraw this result from average.
The problem that when you only has one Purchasing order system will compute a score, let's souppose 80. When you do a reversal of goods receipt, system defines again the score of 80 for this PO and then try to withdraw a score of 80 from an avarege of 80, what will not change the avarege score.
If you have 2 PO with different scores for a vendor/ material, then you Check that the score is withdraw from the average score.
A class room with X pupils. The overall (mean) score for a math test is 80. Later the teacher finds out that one of the pupils cheated and so he decided to remove this score and recalculate the class score. As it turns out the pupil in question had received a score of 80 and thus the removal of this score does not change the class score. The teacher doesn't know how many pupils wrote the test however and thus can not be sure that the score shouldn't infact
return to 0.
Exactly this is your situation; LIS/Statistical-Data is by definition condensed data and not a duplication of all information available.
Pieces of information that are not available are, for example, the number of GRs to a given PO item or the number of times a vendor has been evaluated. Without these, the system can not know that the evaluation to be reversed is also the first and only evaluation for the vendor.
If you wish to have an evaluation which does not consider this score then you can make a reconstruction of your statistical tables (S012) using transactions OLIX and OLI3 and not include the document that the goods receipt was based.