Probably the same way people assume the C's could take the Hawks to six games: blind optimism with little basis in reality.
I'm hearing an echo from your autumn assessment of our likelihood of making the playoffs 
To be fair, I don't think anyone predicted the bottom of the East being THIS bad. The fact that teams like the Celtics, the Heat, the Nets, the Hornets, and the Pacers are theoretically still in position to make the playoffs whereas one of Russell Westbrook or Anthony Davis is guaranteed to miss them is kind of a joke.
Not saying I disagree with you overall. However, the second Paul George broke his leg it became apparent that the Pacers were going to be very significantly worse this year. They were a pretty average team for most of last year and had lost 2 of their best 5 players and having West get a year later into his mid 30's did not seem very promising.
I think the Nets also are probably about where most people would have guessed. They were losing Pierce who was a very solid contributor and also had two of their key contributors in Johnson and Williams being on the wrong side of 30.
The Heat have definitely fell beneath expectations, but I think many questioned the wisdom of bringing back a team that lost the best player in the game and had several rotation players ranging from old to ancient (Wade, Anderson, Haslem). They probably should have been closer to 500 than the 36 win team they will end up being.
However, the Celtics and Hawks have also wildly exceeded expectations. The Bucks have slightly exceeded them. I think the Bulls, Raptor, Cavs. and Wizards were within their expected range of outcomes.
I guess I am just trying to think aloud to figure out how three sub .500 teams could make the playoffs...