Sushil,
I went ahead and wrote to Oliver Lehmann himself and asked for his explanation. Here is what he wrote back:
Once the scheduled finish day has passed, a successfully finished project has an SPI of 1.
It was scheduled to be finished by 100% and has achieved 100% completion. 100% divided by 100% is 1.00.
If a finished project has an SPI >1.00, it has either been finished ahead of schedule and the scheduled finish date lies still in the future, or it must have been cancelled.
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Mit freundlichen Grüßen – Kind Regards
Oliver F. Lehmann, CLI-CP, PMP