Why is the default setting for XP and 2003 "Custom"?

For Windows Server 2003:

Performance Options

Adjust for best performance of:
( ) Programs (*) Background services

Adjust for best performance of:
( ) Programs (*) System cache

Paging file size for selected drive
(*) Custom size:

For Windows XP:

Performance Options

Adjust for best performance of:
(*) Programs ( ) Background services

Adjust for best performance of:
(*) Programs ( ) System cache

Paging file size for selected drive
(*) Custom size: