For both auto-x and road racing, there are turn-in points, apexes, and track out points.
Auto-x will have about 40 of these within a minute.
Road racing has about 8-12 of these every 2-3 minutes.
Road racing traffic to contend with. The drivers senses are not only working on the line, but also the traffic around him, AND what the car is doing at any given point in time. That's a lot of brain processing for 20-30 min per session. Sometimes, the brain can only process 'so much'.
Auto-x doesn't. Driver only concentrates on the line, and what the car is doing... almost a state of hyperfocus. Since each run is less than 1 minute, the driver's brain doesn't get that overloaded.
Since things happen so fast in auto-x, but one can do 10-15 events year, a good driver becomes much more attuned to the car and what it is doing at any given point and starts to make minor adjustments unconciously. This translates very well to road racing in taking an 'off' line, and dealing with traffic to the point of not having to 'think' about it.