My experience is that the DVD-A time estimate is pretty close to useless. I don't know whether it is an acknowledged bug. It certainly isn't a very well-done feature.
What I do to get around this deficiency is use the bitrate calculator at VCDhelp (now DVDrHelp). Here is the direct link to the calculator:
Type the total time for your project and it will tell you the average bitrate at which you need to encode to just fill up a standard DVD-R or DVD+R disk. The default audio bitrate of 224 seems to give pretty good results for AC3 audio. If you have lots of fancy menus, you will need to encode at a slightly lower bitrate.