That's essentially it. The further you move the plug away from the center of the chamber, the more timing required. Look at a hemi. Has virtually zero quench yet runs on 30-32 degrees total. That's spark plug location.
When the plug is up the hole like that, you are essentially retarding the timing. The tests validating this are on the web, published by guys like Dr. Jacobs, who developed the Jacobs ignition systems and many others.
You could also call Champion, NGK, Autolite and ask them. Just going from a standard tip plug to an extended tip (if you can fit the extended tip in there) can be a 1-2 degree reduction in total timing.
Running the plug that far up the hole could easily be 4-6 more total ignition, unless some other factor is involved, like so many guys who love to run a plug that is 2 ranges too hot.