Ronald Sole has given an excellent answer to the specific question asked, but both sentences suffer from the same potential ambiguity.
Old score = 100
Score at end of month 1 = 80
Score at end of month 2 = 80
There was a material decrease (20%) relative to the same base in both months. Either sentence reflects that.
Old score = 100
Score at end of month 1 = 80
Score at end of month 2 = 64
There was a material decrease (20%) relative to different bases in each month. Either sentence reflects that.
But those are two different situations entirely.
For the first situation
There was a material decrease in the scores at the end of the first month, but no further material changes at the end of the second month.
For the second situation
There was a material decrease in the scores at the end of the first month, and a further material decrease in the scores at the end of the second month.
Now this answer will be pettifogging if this text supplements a table or a graph. In that context, either of your sentences will be interpreted correctly. If, however, you are trying to provide a purely qualitative summary of something that is inherently quantitative, either sentence is subject to misinterpretation.