Almost all children learn to talk without formal lessons yet it takes several years for them to learn to read and write in school,not to mention the corporal punishment many suffered.
As for arithmetic, not many men have trouble laying a bet nor children knowing how many sweets can be bought for 20 pence…
How my heart sings 
Strange….Indeed! 😉
They understand how to use numbers but not the notation I guess..scribes of course were honoured men once.