The sharp reduction in the death toll, from 86 to 68, added to a list of police missteps. Officers took 90 minutes to arrive after the first shot was fired at youth gathering for a political party retreat on an island resort.
People who called emergency services from the island have reported being told by operators to stay off the lines unless they were calling about an earlier bombing in the capital carried out by the same attacker.
Police also raised the number of dead in Friday's bomb blast in the capital's government quarter to eight, from seven. The total number of people confirmed killed in the twin attacks is now 76, down from 93.
There was no indication that any specific person was erroneously reported to have died.
Police blamed the mix-up on the chaotic situation that erupted on Utoya island when police and rescue workers tried to sort the injured from the dead under mounting pressure to reveal that the number of slain youths was much higher than originally reported.
"It could be that some were counted twice," police spokesman Oystein Maeland told reporters at a news conference in Oslo, explaining that bodies were spread all over the island and some of the dead hidden under other bodies. "But it was necessary to get that information out there (fast) because it was sky-high compared with the number that media had been given."