Hard truth: It also heavily depends on how good you are. The salaries talked about are usually the top 10% maybe 20%. Like people throwing around Facebook and Amazon salaries. They only employ a small percentage of devs.
That leaves at least 80% that earn considerably less and 50% that earn below the mean.
Quick math lesson: 50% earn below the median. For salaries (in general, I've no idea what it's like for developers in particular) the right-hand tail (high performers with extremely high salaries) and the lack of left-hand tail (no salaries are negative) shifts the mean (arithmetic mean a.k.a. average) upwards so it's actually (usually) going to be more than 50% of population earning below the mean.
Well, it's more of a linguistic/english lesson. I meant the median and just forgot that average and mean are synonyms. But thanks for pointing that out.
I actually edited out exactly the same argument that you wrote, because I (and you) have nothing to actually back it up.
Counterexample: if 60% of people earn very close above the average and 35% earn 30% below average and 5% earn (on average) double the total average, then the majority earns above average even tho the right hand-tail gets high. This is just one possible scenario that is not even completely unrealistic.
Also remember that most crazy incomes are not salaries.
That leaves at least 80% that earn considerably less and 50% that earn below the mean.