The new Rune Memories quest I think it is, gives players access to a Pure Essence Chest.
The amount of initial essence in the chest is based on a formula found on RS wiki, and with 99 Runecrafting, you can recieve 24-25k Pure Essence from the chest.
After quest completion, as you level you Runecrafting skill, additional essence will be provided at each level up equal to Essence of 5x your new level, i.e. if I level from 59 RC to 60 RC, I recieve 300 Pure Essence, and then from 60-61, 305 Pure Essence, and this stack so from 59-61, I would recieve 605 Pure Essence.
Hope that clears this issue up, this is meant to happen.