What happens when you already have 999 arrows in Tears Of The Kingdom? 

aaaaaa you're right Aight I'll waste all of these to reach 420

No idea why 😅

I suspect it's related to the same reason the stores in real life price things at 99.99, or 9.99. Stores do it to make you feel comfortable with your decision to purchase, because it's only 99.99, not 100.00 (plus tax). It's a psychological thing, but in this case, it may work in a way that keeps you wanting more. See how it intrigues you, and the video creator? See how you always want to be at the max? It could also be something as simple as them not wanting to cross into 4/7 digit numbers(depends on the item in question); and that gets into like coding/memory theory. Saving space n such.