Compact does not appear to be working

After indicating YES to compact, restarting my machine and re-executing CRC, it indicated there was still 0.3 wasted space. about 250 KB.
I am on Vista Home Premium, 64 bit sysstem HP with 4 gigs memory.
Any Ideas? Does compacting only take place when the the registry is fragmented more than a critical amount?
Thanks,
Miles