Maybe that one time your Owlbear just didn't get many criticals or had more misses or something. There is definitely a noticeable range. For instance Sadiele killed the wolf knight in 1:23 one time, but took as long as 2:14 another time. Rovegar's best time for me was 3:23, but his worst time was 4:51. So the range between fastest time and slowest time could be over a minute depending on the hireling. That's why I tried to run each scenario at least three times to get a better idea of the average time and a stab at what the range from fastest to slowest might be.
If we ran them 20 times for each hireling we'd probably have a pretty accurate (within 10 seconds I bet) idea of the average time for each hireling (I probably won't do that though).
I really appreciate that you took the time to test it.