I think it has to do with reading speed, volume and fatigue, and less likely training. Some non-neuro trained private practice rads read normal/routine head CTs in 1 min (max 2 mins). I read them in 3 mins (and up to 6 mins for more complicated/post-op cases) since I like to look at the ENT structures and subtle areas/blind spots. I have seen the routine 1 min reader miss a single small lytic lesion that returned as a plasmacytoma 2 months later. If this doc spent an extra 1-2 mins on the scan I doubt the lesion would have been missed. However these ER/inpatients shifts are extremely busy so they do what is needed to stay above water.
During training, I would see neuro CAQ attendings spend 5 mins or more reading a routine head CT, checking the chart, etc. Easy to do this since they have trainees and rarely have to answer the phone/deal with frequent interruptions. When on private practice ER/inpatient shifts at busy places, there is rarely any time to look at the chart and as such they read studies with indications like "stroke", "stat", "r/o patholigy"[sic], etc.