What is so unusual about it? if there had been 50 people its quite like say 40 of them would have exhibited the exact same symptoms and timescales while the other 10 were widely different.
What's unusual is that both people in one incident decided to "break" the "rarely any consistency" argument. And with a (allegedly delayed) timing so uncanny, that neither of them could call for help. That same uncanny timing was surprisingly absent in the case of Dawn and Charlie.
Plus let's not forget one very important detail - the "delayed action nerve agent" argument. Dawn Sturgess succumbed immediately.