I have heard from multiple suppliers over the years that their estimates are those provided by their sources, often the manufacturer/Hasbro. They aren't often given updates until it's too late. I've seen this on other big sites like Robot Kingdom as well.
I totally get that doesn't help us, the consumer, when we're trying to stack items into a shipment to save on postage. I've been stung a few times myself.
One of my favourite things about BBTS was the extreme Pile of Loot Period, having it be only 2 months instead of 6 doesn't help with their appeal to folks like us. they must have been renting a second warehouse to hold all their POL product.