My theory is that buying/crafting arrows increases your max capacity for that arrow type while using the replenish skill would fill your stock up to your current max capacity for whatever arrow type you're replenishing. I'm basing this off the fact that in one of the videos a guy showed his inventory and it listed his current bronze arrow count as 94/99. This means if he used replenish, he would be back up to 99 (or for example if he had had 54/59, it would've taken it up to 59 with 54 being your current amount of arrows and 59 being your "max capacity" from arrows bought/crafted so far)
We'll just see later on I guess