"As long as it's better than the average driver, it's an improvement."

Careful- this is true if interpreted juuust right, but it's subtle.

Ultimately, you care about, roughly, the net decrease in Bad Things (injuries, deaths, property damage, weighted however you want) (and possibly also the net increase in Good Things (people being able to get places, mostly; otherwise, an excellent FSD system is one which simply stays in park and refuses to move!)) that results from the deployment of an FSD system.

That looks roughly like the sum, across all trips, of "expected Bad Things if FSD was available as an option for that trip" minus "expected Bad Things if FSD was not available".

The key here is considering each trip as a choice between FSD and non-FSD and /then/ averaging. You can't just say "FSD's per-mile accident rate is lower, so it's safer"- that misses a lot of important effects!

For example, consider selection effects related to weather and road conditions. It's easy enough to imagine an FSD system that falls apart entirely in bad weather, but works alright (but worse than a human driver!) in good weather. Let's say it's got 0.1 Bad Thing Points Per Mile (BTPPM) in good weather and 100 BTPPM in bad weather. But human drivers also do worse in bad weather, naturally- maybe they've got 0.05 Bad Thing Points Per Mile in good weather and 10 Bad Thing Points Per Mile in bad weather.

Now, assuming the poor performance in bad weather is obvious to drivers, drivers will only activate FSD in good weather. So the measured BTPPM for FSD is roughly 0.1. Human drivers, though, still have to drive in bad weather sometimes - so you might see, say, 1 BTPPM from human drivers.

Even though human drivers are better than this hypothetical FSD system in all conditions, they look way worse!

And that's just one effect- here's a related, but distinct one. Take the same example from before, but now the FSD system matches the human in good weather and bad weather (0.05 and 10 BTPPM) and, in awful weather and road conditions, where the human might have 1000BTPPM, FSD manages just 100BTPPM.

The FSD system's performance matches or is superior to the human in all situations. Surely this leads to a net decrease in Bad Things?

Not necessarily- what if people take trips in awful weather at 100BTPPM which they wouldn't take with a human driver at 1000BTPPM? (And this assumes people aren't lulled into a false sense of security because they know FSD is "safe", leading them to underestimate the risk in awful conditions). This is where you have to weigh in "how much benefit does taking this trip in the first place provide".

And one more thing: remember you have to consider the vehicle. Any vehicle with FSD can and would almost certainly (if it's human-drivable at all) provide very sophisticated ADAS features when driven by a human, and would likely be new enough to have quite modern safety features. So you have to remember to not just think about "FSD accident rate per mile v.s. human"; you do have to make sure to compare within the same vehicle.

"Better than the average driver" could be interpreted to take all this into account, but the straightforwards interpretation ("lower accident rate on average") doesn't, and can be surprisingly misleading.