Therapy Hours vs. Therapy Usage on M-Series

What is the difference between these two? Therapy Hours seems to be the amount of time I have the machine/mask on (ex. 8.9) but that's just a guess. Therapy Usage shows Hrs/Night at the top of that screen but the number doesn't seem to relate to anything (ex. 6:42). The numbers I listed were from the same night. Also, I don't take the mask off at all in the night and only turn the machine on or off when I go to bed and get up in the morning.

Any thoughts?