What is special about the 73-88% Barrows reward potential range?

Solution 1:

After doing a bit more research on the barrows wiki page. It states that:

If players wish to receive more runes (which are generally worth more coins) instead of bolt racks and key halves, they should aim for a Reward Potential of 880. Having exactly 880 points maximises the quantity of blood runes without having bolt racks take up reward slots.

So this accounts for the max of 88%. But what about the 73%? Well, further down the page it states that the roll for blood runes:

Requires 756 reward potential to roll.

So 73-88 is essentially the best range to have to get the max amount of mind, chaos, and death runes, while also still guaranteeing a roll for potential blood runes. At 88%, you can receive the max amount of runes possible (if you are so lucky). After 88% (exactly 881 potential) you will roll for bolt racks and lose a potential reward slot for runes.

Just as a quick guide, these are the potentials needed to roll for each rune/bolt drop.

1. Mind runes require 381 reward potential to roll.
2. Chaos runes require 506 reward potential to roll.
3. Death runes require 631 reward potential to roll.
4. Blood runes require 756 reward potential to roll.
5. Bolt racks require 881 reward potential to roll.
6. Key halves require 1006 reward potential to roll.
7. Dragon med helm requires 1012 reward potential to roll.

Note: The chance for a piece of barrows equipment is "unaffected by the rewards potential that is built by killing other monsters in the crypt."