The reason why it is omitted is the age of Saul and the number of years that he rules OR the words were added after it.
This problem is very old because the verse was already omitted in some versions of the LXX.
So a problem of misunderstanding or text corruption. Some translations using other numbers or even use __ because they have no solution for the problem.
I found a solution for the age and years problem.
1Sa 13:1 Saul reigned one year; and when he had reigned two years over Israel, (KJV)
1Sa 13:1 reigned one year: Heb. the son of one year in his reigning (KJV footnote)
The Hebrew idiom "son of a year" is not in English but means year of origin or first year.
1Sa 13:1 It was a year since Saul began to reign and he had reigned in the second year over Israel, (my translation: the RHB)
In the first year was Saul called a king but it takes a year before he was recognized as king over whole Israel.