As far as I remember it each city ( well certainly ORD) certainly also had a dedicated "terminator". There's probably a couple of reasons for the additional LHR-ORD-IAH flight...
1. The load on the route (i.e. number of passengers ) was not enough to economically run all flights as separate flights to each destination
and
2. It uses one London slot to serve two US cities.
As you imply this is no longer the case and
AFAIK each city gets dedicated services.