This is a common problem with automatic recording of engine parameters. Manufacturers are normally only interested in the 'eyeball' figures as there are errors normally associated with the airborne data gathering.
Most data gathering systems will only record figures to 12-bit accuracy, this leads to numbers being rounded up (or down) depending on the installation. Without going into the boring stuff, if your engine is doing 29,994 RPM gas generator, the recorded number might be anywhere from 29,982 to 30,006 RPM.
I know what you are going to say now "How come they don't accept those figure when my mark one eyeball is not accurate enough to tell the difference from 29,960 to 30,000 even on a good day?"
Another problem is hysteresis of the cockpit indicators. This is taken into account on the graphs, but the onboard equipment does not. Figures taken during a transistion where the engine is accelerating can mislead the automatic algorithms and give a fail.
In the end, the engine manufacturer always wins. After all, it's their train set, and your lives.