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Yes, bucketing is a reality.
Remember school and the "Curve" Grading system? Well, bucketing is much the same. It compares you to your peers....and peers only means those you are similarly compared to.
FICO uses comparative analysis. It uses the law of large numbers. Simply put, they take segments of borrowers based upon identifiable credit characteristics and then analyzes under what circumstances do relative outcomes occur.
So, when you are bucketed to one particular segment, your grade (FICO) is the curve grade of that bucket (class). When you graduate to another bucket (class) you are graded against your new bucket peers, which means that even though you improve your overall credit situation, you score lower because you compare lower to your new bucket.
Look on the Understanding FICO Scoring thread. There is many, many, posts about this.
Try searching "score buckets" "FICO buckets" etc. All different variations.