The good news is that unpaid item reminders disappear without long-term consequences and/or ramifications as soon as the item is paid. Your sellers are not lying to you; it is a setting which anyone on ebay.com may select if they want their payment reminders sent automatically by the system. It is up to the seller to decide if he or she will enrol in this timer-saving-touted feature offered by ebay. The seller of the other auction of which you speak (after eight days) may not have subscribed to it. Don't take it personally.
I would suggest in future that if you know there is going to be a delay between the end of an auction and the submission of payment, tell your seller ahead of time. As a buyer, you might be surprised how many items really go unpaid and how incredibly disruptive it is to a seller's ability to conduct timely business.
Good luck as you carry forward.