"Injuries were not significantly related to race running speed, training surface, characteristics of running shoes, or relative weight."
Perhaps, somewhere, Dr. Marti did in-fact express surprise about "the most common variable" — whatever that means.
However, at-best Christopher McDougall seems to have chosen not to report conclusions that could have helped some runners avoid injury.
Marti, B. (1989). "Relationships between Running Injuries and Running Shoes — Results of a Study of 5,000 Participants of a 16-km Run — The May 1984 Berne Grand Prix". In Segesser, B.; Pförringer, W. (eds.). The Shoe in Sport. pp. 256–265.
https://archive.org/details/shoeinsport0000unse_r2y3/page/25...
In the 1984 Bern 16 km race questionnaire, runners who had no shoe brand preference and presumably changed brands frequently had significantly fewer running injuries. There was also some correlation between higher shoe price and increased injury. "It is probably incorrect, however, to interpret this surprising finding to mean that more expensive shoes cause more running injuries…". That group was 1 1⁄2 minutes slower than expected from their training and had a higher proportion of orthotics use. It may well be that runners with existing injuries hope that expensive shoes will fix their body.
How well does the reporting fact-check?
From "Born to Run":
"But what surprised Dr. Marti … was the fact that the most common variable among the casualties wasn't training surface, running speed, weekly mileage, or "competitive training motivation." It wasn't even body weight, or a history of previous injury: it was the price of the shoe." page 172
From Dr Marti's report:
"Occurrence of jogging injuries was independently associated with higher weekly mileage (P < 0.001), history of previous running injuries (P < 0.001), and competitive training motivation (P = 0.03)."
https://journals.sagepub.com/doi/10.1177/036354658801600316