為何香港的vaio z27不能寫double layer dvd, 而較平宜的z26可以?

http://www.sonystyle.com.hk/ss/product/vaio/index_2.jsp
z27 的optical drive spec:
Optical Disc Drive Drive Blu-ray Disc™ Drive
Maximum Reading Speed BD-ROM: 2x/
BD-R: 2x (Single Layer), 2x (Dual Layer)/
BD-RE: 2x (Single Layer), 2x (Dual Layer) /
DVD+R: 8x (Single Layer), 6x (Double Layer)/
DVD-R: 8x (Single Layer), 6x (Dual Layer)/
DVD+RW: 6x/DVD-RW: 6x/
DVD-ROM: 8x/DVD-RAM: 5x/
CD-ROM: 24x/CD-R: 24x/CD-RW: 24x
Maximum Writing Speed BD-R: 2x(Single Layer), 1x (Dual Layer) /
BD-RE: 2x(Single Layer), 1x (Dual Layer) /
DVD+R: 8x (Single Layer)/
DVD-R: 8x (Single Layer)/
DVD+RW: 4x/DVD-RW: 4x/
DVD-RAM: 5x/
CD-R: 16x/CD-RW: 10x

沒有提能寫dvd DL, 為甚麼這樣的, 較平宜的z26 反而可以?? z27是漏寫還是真的不能, 以下是z26 optical drive spec:
Optical Disc Drive Drive DVD±RW/±R DL/RAM Drive
Maximum Reading Speed DVD+R: 8x (Single Layer), 8x (Double Layer)/
DVD-R: 8x (Single Layer), 8x (Dual Layer)/
DVD+RW: 8x/DVD-RW: 8x/
DVD-ROM: 8x/DVD-RAM: 5x/
CD-ROM: 24x/CD-R: 24x/CD-RW: 24x
Maximum Writing Speed DVD+R: 8x (Single Layer), 4x (Double Layer)/
DVD-R: 8x (Single Layer), 4x (Dual Layer)DVD+RW: 8x/DVD-RW: 6x/
DVD-RAM: 5x/
CD-R: 24x/CD-RW: 16x