DRS in full data synchronization, will be a large number of tables sharded multi-thread concurrent synchronization to improve efficiency, under normal circumstances the table will be sharded according to the primary key, and the default shards each shard 520000, once Bob found during data synchronization, there is a large table with a primary key T1 was divided into many shards, However, some shards have more than 5200 entries , and some shards have less than 520000 shards, which of the following options is the possible reason?