Explanation/Reference:
A: It could be that row migration.
D: It could be that row migration or chained rows could cause it.
Note:
* Some facts about scans:
Scans exists in "OLTP" systems
Exadata smart scan requires a direct path read.
A direct path read is chosen at runtime based on internal heuristics
The STORAGE clause in an explain plan doesn‟t necessarily mean you will perform a smart scan.
* The buffer caching in certain "OLTP" environments can occasionally induce conventional reads when smart scan is faster.
* Typically see cell multiblock physical read instead of cell smart table scan waits
* No one-size-fits-all solution can be given here but it is very fixable.