Once again checking prices

Misfits series 3 will be released on Blu-ray on January 12 and Amazon are selling it at £15.93. But wait I could buy all three series for £33.69 or all three individually. Hmmm?

Series 1 £8.99
Series 2 £8.99
Series 3 £15.93
Total £33.91

Series 1 & 2 £12.49
Series 3 £15.93
Total £28.42

Series 1-3 £33.69

So it's only slightly cheaper to buy the pack than to buy them singularly; but it's even cheaper to buy the first pack and the last series by itself. How does this make any sense?