Whose car was it? What year and ITR number, and what market was it first in?

Are you aware of any tuning history? Is it outfitted in any way unusual? You said you couldn't see anything unusual in the motor, but what about the rest of the car? Is there any sign that it might have been used at some time for anything other than normal road use?

Many "blueprinted" motors were tweaked a bit more than the lower FIA Gruppe N specifications actually allowed, and nobody ever discovered it because the motors were never torn down. Higher Gruppe N numbers would allow quite a bit of modification that would not be externally visible.

A normal road car could have several different kinds of OEM pistons installed that would increase the compression.

A Spoon or Mugen computer could be used, and it would not look any different from the OEM computer.

Changes to the cams, intake manifold and throttle body are popular modifications that would not necessarily be obvious to the naked eye of a dilettante.

It's also entirely possible that the dynomometer was calibrated incorrectly.