PO Line item deletion not posible after GR cancellation without ERS Settlement run.why & how to avoid?

I was getting an error message "Evaluated receipt settlement open, deletion not possible" in R3 (message no ME867) when trying to delete a line item in PO. We did a GR and than reversed it (102 movement) after that I was trying to delete a line item. At last I ran the ERS settlement & then I am able to delete the line.

My question is why do I need to run the ERS Settlement. We run custom ERS settlement program every night. So during the day if the user does a GR & immediately cancells the GR why does he need to wait for the ERS settlement run for PO Line item deletion. My question is why we need to run ERS settlement & is there any way to avioid that if i want to delete an Item.