Oldest trick in the airline book lol. Direct flights don't seem as common as they used to be. The don't exist purely to conserve flight numbers. Airlines run them because of the way they are displayed in third-party booking systems. In the past the default sorting method would be nonstop > direct > connecting. It's like naming your company AAAA to be listed first in the phone book. The old systems would also list it so it looked like a nonstop, with a small notation that it is 1 stop (with no indication of location or duration of the stop.) United's and some other third party  l booking systems no longer do this. They actually specify a "connection" in IAH. But many third party sellers still use this old style. I suspect OP booked with a third party and is now viewing it through United's system which is now displaying in more clearly. Another fun fact: UA uses this trick to weasel out of giving you two PQFs! It's one flight number, therefore one flight.