The CAS said last week that there was "insufficient evidence" of anti-doping violations against 28 Russian athletes banned for life by the IOC as part of its investigation into doping at the Winter Olympics four years ago.
"For 28 cases, the appeal was accepted and our decision annulled," IOC member Denis Oswald, who led the commission, told an IOC session in Pyeongchong on Tuesday.
"It was a shock as we felt the evidence we presented was strong enough to justify the sanctions we had taken," he added. "I have difficulty explaining it because I don't understand it myself."
The IOC has banned Russia from the Pyeongchang Winter Olympics starting later this week over "systematic manipulation" of the anti-doping system in Sochi but 169 athletes with no history of doping have been invited to compete as neutrals.
Oswald said CAS arbitrators had applied criminal standards of proof that made it far more complicated to prove wrongdoing. The IOC is considering whether to appeal the CAS decision at the Swiss Federal Tribunal.
"They applied criminal standards where the first doubt you have does not allow you to sanction," he said.
After the CAS decision, Russia's Olympic Committee requested that 13 active athletes and two who had become coaches should be allowed to participate in the Feb. 9-25 Games but the IOC has refused to extend invitations to them.
(Reporting by Karolos Grohmann; Editing by John O'Brien)
