Blu ray quality control

[tvmog]

Have just watched "Inglorious Basterds" on blu ray. For various reasons I had to watch half of it on one disc and half of it on another. The picture quality on the first disc was excellent - vibrant colours and sharp detail. However, when I put the second disc in the picture, although still very good, was obviously softer and the colours slightly muted. They were both Blockbuster rental copies so I suppose it is possible that one was in better condition than the other, but is that likely to make a difference?

 

[Liam19]

What are the 'various reasons'?