If there's a delay processing a getMore for the oplog it's possible the heartbeat time will be later than the _slaves time (shown in local.slaves). _slaves potentially could be updated from the heartbeat optime if > the optime found from getMore(), or wtimeout could also take the heartbeat optime into account if > _slaves optime.

Test demonstrates if you insert a 10s delay, for example, before the processing of getMores()