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Credit scoring is not a uniform algorithm for a given catedory for all consumers.
Credit scoring uses a concept called scorecard categorization that develops different treatment of consumers based on a categorization of some aspect of their credit file, and then uses different algorithms that compare you with those subsets of consumers that have that similar categorization.
That categorization process is commonly called bucketing.
For payment history, as long as you have any remaining major derogs, you are scored in a less-favorable category, or bucket, which is commonly called a "dirty" file.
You will normally not see significant score improvement until your file becomes "clean" of all major derogs.
Thus, deletion or exclusion of a collection or CO will normally not have as much impact until all other major derogs are deleted or excluded.