There is no concrete evidence that the brain does math. If the brain did select things from a probability distribution, then why isn't everyone a math genius.
No one really knows how it works.
As for your second point, assuming that humans are Bayesian, there are many reasons why people would have variability in their mathematical ability, including different priors and differences in the ability to estimate posteriors.
[0] https://scholar.google.com/scholar?q=brain+bayesian&hl=en&bt...
The brain absolutely does tons of math. It's just well below the level of consciousness. Most people can learn to do analog math problems well within a few percent accuracy. It's symbolic math that is foreign to us.
There are people who never manage to learn to do basic elementary-school math with fractions and percentages and yet manage to bet on sports and balance their checkbook because they're unable to translate their intuitive mathematical instincts into abstract formal math.
If the apple falls predictably from the tree, where is the math genius who plans its trajectory? Does the apple itself know physics?