这是由于分布式集群正在迁移数据，它导致count结果值错误，需要使用aggregate pipeline来得到正确统计结果，例如：
db.collection.aggregate([{$group: {_id: null, count: {$sum: 1}}}])
引用：“On a sharded cluster, count can result in an inaccurate count if orphaned documents exist or if a chunk migration is in progress.”
参考：http://docs.mongodb.org/manual/reference/command/count/
