With the DVD technology, a small gap is left in the layer 1 track every time it crosses the layer 0 track (twice per revolution). As a result, the data capacity of layer 1 is less than layer 0 which means that a dual layer disc only has a capacity of 8.5 GB instead of the 9.4 GB that might have been expected.
In the case of Blu-ray discs, it was realised that the gap was unnecessary as the laser is perfectly capable of focussing through track 0. Consequently, dual layer Blu-ray discs have double the capacity of their single layer counterparts (50 GB as against 25 GB).