Rebuilding an index might reduce the number of leaf nodes by about 20% - 30%. The most you can possibly expect from this reduction is 20%-30% for very expensive operations like a FULL INDEX SCAN. The typical INDEX UNIQUE SCAN gain of an index rebuild is 0%-2% because the depth of the index is not reduced by the rebuild.
In my understanding , the btree structure/shape is almost uniqe. why would rebuild the btree reduce leaf numbers ?
is there any uncertainty around the btree build process ?