A 42-year-old pesticide factory worker presents with acute cholinergic crisis 30 minutes after suspected organophosphate exposure. Atropine is administered with good response to muscarinic signs. However, 6 hours later, despite continued atropine, he develops severe muscle weakness, respiratory depression, and fasciculations that persist. A colleague exposed to a different organophosphate compound recovers fully within 48 hours with the same atropine regimen. Which feature best explains the difference in recovery between these two organophosphate exposures?