You're getting confused because there are two numbers. To add to the confusion, they are both estimates. In actuality it works exactly how you think it should.
Your 622 has a 350GB drive, a certain part of this is reserved for VOD downloads and other junk, the portion that's left for your recordings is estimated to hold either 200 hours of SD, or 30 hours of HD, or some combination of the two. The thing that's confusing you is that there are two numbers. Think of it this way. If 200SD = 30HD then each hour of HD uses the same space as 6.67 hours of SD. So if you record 1 hour of HD, then you would see both counters will reduce by 1/30th. So you'll see 193.3SD, 29HD remaining.
The amounts will never be "accurate" because compression efficiency varies from one program to another.