"But what can we, as sellers, do about it, - apart from not entering tracking or marking stuff as dispatched?"
I add the RM 2nd class reference number as "tracking" for the convenience of my buyers and also send them an ebay message telling them that their parcel is in the post. Usually the day after they pay and in the hope it will jog their memory if the parcel arrives "late" (by ebays standard), they might remember that it was posted Next Day and realise I am not at fault.
I tried not adding tracking until day two of my dispatch window a couple of times. Nothing changed with subsequent sales FDDs. Which again led me to question how 'dynamic' the calculation really is and what it is really taking into account.
The only other explanation I can think of is that when I enter the reference number on ebay, I'm told to wait while the info. is verified. Is part of the verification the date the number was issued at the post office? If so, it doesn't matter what day it's uploaded, it registers with ebay as the date I posted -- so back to square one and a waste of time.
So I reverted to uploading on the first day of my dispatch window -- preferring to put my trust in my buyers common-sense when ebay demonstrably has none and churns out the same nonsense whatever day I upload tracking.
As I'm a low volume private seller my account assessment is over a rolling twelve months, with an allowance of 5 late delivery defects at any one time. I make the obvious choice during winter weather, strikes and back-logs, as any sale is quite likely to result in a defect when ebay insists I should get a parcel from one end of the country to the other in two days.
e.g I sold an item (located in Kent) to a buyer in Argyle on 29/11, dispatch by 1/12, FDD shown to buyer 2-4/12. It still hasn't been delivered according to tracking, possibly due to heavy snow fall, then snow melt and flooding.
I ended all my listings on 1/12 and won't start listing again until about mid-January, assuming the weather isn't too bad.
