DVR Space messed up

I have a newly installed DVR which is suppose to have a 500 gig hard drive.  With everything deleted it says 993 Sd or 363 Hd available.  When I record 1 hour of HD it goes to 89% available 884Sd or 322Hd. I only recorded 1 hour of HD.

With 34.5 actual hours of HD recorded it gives 82% available 815Sd or 298Hd.  I must be some kind of new math.  Where I am from 363 - 34.5 is 328.5 or 90% available.  Is anypne having the same problem?  Thanks.