Isn’t the explanation just that delivery estimates have a wide margin of error? To guarantee delivery by a target date the order must be placed by the beginning of the margin, but if you order within the margin there’s some probability of getting that date by chance.
To explain the two estimates days apart both returning the same Sunday, consider that the week is heterogeneous; maybe some regional hop is available specifically on Saturday regardless of how early you order.
Sure, but the regulator in this case is making an argument that it's creating an unfair pressure to make you purchase a thing, and that the timer is consistently shorter than it needs to be.
It's the same thing as going on a website and it says "order within next 30 minutes for a 50% discount" and then you come back an hour later and it still says the same thing - it creates an incentive on you to purchase by creating an illusion of urgency. It's the illusion part that the regulators have a problem with.
To explain the two estimates days apart both returning the same Sunday, consider that the week is heterogeneous; maybe some regional hop is available specifically on Saturday regardless of how early you order.