Variable or constant bitrate for recording

[stdim]

Hello,
I was experimenting with recording capabilities and I wonder

1) What is the usage of High quality and standard quality profile ?
When you select a value for compression ( lets say 4500 Kbps)
is it constant bit rate or variable ?

I have noticed that in
WinTV 10 minutes recording using a profile of variable :4800 Kbps and Max:6000 Kbps gives an mpg file 400KB
in GBPVR 10 mins             -"-           high 4300 Kbps, low 4300 Kbps gives a file 433 KB

2) why higher bit rate in WinTV gives smaller mpg file than GBPVR ?...
   is it something I do wrong?...
Thanks...