I'm not sure I understand your point.
79 NM = 91 SM.
The ground speed can and will easily change over 91 miles. In fact, 10% isn't that big of a deal...3.0 miles per minute to 3.3 miles per minute.
The problem also is a mile or two or three can easily be added or shaved off depending upon precisely where 305 was at the time it was near the Eugene VOR. Was it just past, just before, directly over the top?
Trying to get precise to this degree is impossible. Way too many variables. We need to look for glaring flaws that cannot be easily explained.