A while ago bitcoin clients changed from facoring the 'longest' chain to favoring the chain with the most work done on it. (To prevent long chains with low difficulty)
So... the consensus rules of the network changed, you need to make sure you have the correct client, and bitcoin is weakly subjective after all?
Asking "what's the correct client?" will always be a subjective question
Bitcoin doesn't decide what is called bitcoin, we as a community do
In practice this was essentially a soft-fork. But yes the consensus rules of bitcoin software changed.
The client can choose properly, but it needs to "call a friend" in order to get the options - if the client doesn't receive the proper chain but only fake ones, it will chose the fake one with the most work done on it.