The available time shown when the drive is empty seems to significantly underestimate the actual time available. With my original drive, it showed 8:14 available. I did not fill that drive up, so I don't know what it would actually do. With my 120GB drive, it showed 12:36 available when empty, but I filled it up with 14:53 of recordings.
I think what you are seeing is the effect of the underestimation of the available time, not the usage of the space above 137GB. Empty, your drive showed 14:30 available compared to the 12:36 mine showed when empty. That is proportional to the 137GB limit/ 120GB. What you are seeing as you fill it up is that you're getting about 17 hours, which is still maintaining the 137/120 proportion to the actual 14:53 that I got when I filled my 120GB drive with recordings.