Making up time during flight for late departure

Hi.I'm reading an accident report on the SQ flight that had a tailstrike at NZAA earlier this year, and the reviewer makes the following comment:"The length of the delay (13mins) would have concerned the pilots somewhat because the time loss would not normally be able to be made up during the flight."My question is, why is it not "normally" possible to make up a 13min delay during a 9hour flight from Auckland to Singapore?Anyone have any reasonable answers to this? CheersAllblack