Shi/Yon and Shichi/Nana - Explained..

Okay I'm sure a lot of you will be familiar with the Japanese numbers of 4 and 7, how they can be pronounced each with 2 different words. My understanding is that Shi denotes 4 and Schichi denotes 7 only when counting under 10. Once you get over 10 you would say 17.. Ju nana rather than ju schichi and 34... San ju Yon rather than San ju shi.

I was just wondering if anyone could explain why this rule exists? And if so do I fully understand it?

Case by case. Yon, nana, jūshichi and sanjūshi are also common.

Japanese numerals - Wikipedia