Here's what I think. I can't say for sure the situations are the same.
I don't use "Prime", and I see the "in stock" explanation on your part.
I suspect that Amazon is selling things they don't have "in stock".
I ordered a master cylinder from them.
Brand new Raybestos for 44 dollars and free shipping.
(I checked a few days ago and it almost 70 now on Amazon)
It took a while to get it. And I think it did give me a distant date.
It was like they had to order one to sell it.
I also suspect you got a very good price on your item.
I'm sure you are aware that Amazon has woken a sleeping giant, as Clark Howard said in regard to Walmart.
I also heard something about brick and mortar grocery story stocks being down last week.
Don't really follow that.