Final 3 Greens is correct. In the early days of T5 there were disconnects between gate and FIDs, though these now appear to be ironed out. At least T5 has gate information and FIDs plumbed into the same system, in many (most?) airports they are on 2 separate systems and rely on a human to get it right. T5 FIDs has some intelligence, so it will say - for example - boarding in 5A for a 5C flight, but say something different in 5C, namely go to gate. Also delays are detailed in the lounge FIDs but short ones are not mentioned on the public FIDs.

In many US airports the FIDs are bulk loaded in advance, it then goes on a timed script through the various stages, completely oblivious to what actually happens at the gate. That script can be interupted, if someone remembers to do it. But, and this is the killer point in the USA, very, very few passengers go to lounges on domestic flights. There is no reason for most passengers to be anywhere other than in the direct line of sight of the boarding gate. That's just how they do it over there.