I really don't know - I looked up several listings (other seller's) that would technically qualify, and some showed the F&F logo while others didn't, and there wasn't any discernable difference (eg postage services selected were the same), other than the dates eBay showed for the ETA. The F&F listings say on or before a date within 4 days, the non F&F show between two dates, one falling within the 4 days and one falling outside of 4 days, so I'm guessing it has something to do with the average delivery time estimate, as I saw eBay say they also take seller's postage history for delivery time into account (in the US, tracking is used to determine that, but in Australia, for non-registered letter rate items, I can only presume they base that on the average time it takes for buyers to leave feedback ? )