Why 1T strorage different with DTV and Tivo

I haven't used a Tivo since HDTV came out many years ago. I moved over to a Comcast DVR, then moved to DirecTV to get a better DVR. It was better; but I am ready to finally return to Tivo. It is the best. My question is about the Roamio Plus with 1T of storage. Tivo says that it will record 150 hours of HD. My DirecTV Genie with 1T records 200 hours. I would have thought that it would be the same. Does DTV use a different compression than Tivo?

The reason I am asking is that I am happy with the 200 hours; but with only 150, I might need to get a larger hard drive. Any help would be appreciated.