Each feed has only certain frequencies covered at a particular receiver location. In a metro area like Houston, not all ATC transmitter locations are in one location. KHOU only has the Final Approach sector for HOU available since the transmitter is close by...aircraft transitioning through other ATC airspace are covered on other frequencies whose transmitters live closer to IAH.
That will explain what you are experiencing - it's the nature of the beast. And when they combine frequencies on the overnight shift it can change. Each ATC facility has different SOPs (Standard Operating Procedures) that affect this.
Hope this helps.