So, questions:

- Is this normal?

Yes.


- Why might it happen?
(Is it innaccuracies of the GPS system, this particular model, or maybe
interference?)

Speed is calculated by using simple time/distance calculations, measuring
the time it takes to get from one known fix to another. It is common for a
GPS to get an errant fix from time to time. Sometimes, these errant fixes
can be quite a ways off, but are usually only momentary. Nevertheless, they
are included in speed calculations. The seemingly unbelievable max speed
results from the very fast side trip your GPS thought you made while
visiting the errant fix. I've seen max speeds in the many hundreds of MPH.
Probably the closest I'll ever get to warp speed in my lifetime.




- If I see that the max-speed is inaccurate, should I also assume that
the other readings (odometer, average speed) are inaccurate?

Inaccurate is a relative term. I would suggest that your GPS is likely to
be more accurate and reliable for the above measurements than any other
instrumentation that you have on board.