Explanation
Distribution works by testing a post on a small audience and reading what happens, then widening if the signals are good. That process needs samples, which is the real reason posting volume matters: four posts a month gives the system almost nothing to learn from, and every post starts from close to zero.
It also explains why engagement rate against followers is a broken metric here. A successful post is one that travelled to people who do not follow you, and measuring it against your follower base punishes exactly the outcome you wanted.
The common mistake
Treating a single post's performance as feedback. Distribution varies enormously per post, and it takes several posts in a consistent shape before anything can be read as signal.