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.