I'd say your scans look fine. Safe to always assume there will be some media variability, so I wouldn't think your fluctuations are highly out of the ordinary. Other contributing factors are the usual quality and consistency of power from your power supply, background apps running, harddisk fragmentation state, cpu/ram resources being used, system/drive/media temperatures, etc.
If you're 'comparing' your scans to others posted, just keep in mind most scans posted here are 'best case' or 'better case' representations of their burns. That's a generalization mind you.