I have just updated the photo to show the correct RPM figures - I hadn't installed the config file for this version of the data logger.
I guess the RPM trace might be a combination of several factors...
Firstly the ECU (Motec) probably does some sort of smoothing (moving weighted average ?) - this can be seen by the rise in revs after the gear change taking a while to start climbing.
Secondly it might take a fraction of a second to start slowing when no more power is being applied...
Maybe someone could enlighten us both
The track was flat - it was on a military airfield in the SW of the UK.
Engine wise - its a turbocharged 1700 Ford Puma.