Posted for Walter Atkinson <walter@advancedpilot.com>:

 Ahh, you've noticed that the sparks don't ACTUALLY ignite at the specified
and set timing?  Good observation.  Few people realize that.  The only way *I*
know of to answer your question is with the mechanism George uses on the
engine test stand to measure not where the timing is SET, but where it
actually fires.  They are not one and the same!
 
 Walter