I would guess that the minute rating is a guess bearing in mind you can burn info at different qualities. They are both 4.7 gb disks so must take the same amount of info.
As example if you burnt MP3 files at 256kps you would get less on it than if you burned at 128kps. Same with video. My stadalone DVD recorder can burn 1 hour at very high quality, 2 hours at high quality, 3 hours at stadard quality etc. I presume thats what they mean.