Image Burn Device Buffer

When I make a backup of a movie using Image Burn I have noticed that the Buffer is 100% but the Device Buffer is going from 86% to 89% and was wondering if they should be both 100% The backup copys come out good but was just curious if this is normal? I have my write speed at 8x This might not be a something to worry about but am wanting to know if this is what it should be. Any insight to this would be great and thank you in advance. If need to ask any questions please feel free to ask. Thank you again.

This is normal.