NEC ECC: Why Core clauses state single digit and sub clauses state two or more digits in NEC3?

Why Core clauses state single digit and sub clauses state two or more digits in NEC3?
i.e. Core Clause 1 - General, but sub clause 10 - Actions, under the Core Clause. Why don’t it use as 1.1.

Because they do!

That is all I have I am afraid. They split the 9 sections of the contract using single digit numbers. Each section within that section is then split into 2 digit numbers, and each clause within that section using a decimal place.

So payment is section 5, split into four different sections 50/51/52/53, and each clause within that section then using a decimal place so 50.1-50.6, 51.1-51.5.

It is what it is. No good/bad reason for it. At the end of the day you just need a reference number to be able to use to quote a clause - however they have done it.