Slideshow was not intended to precisely timing soundtrack and video and you should not rely on this feature. This is something which generally require a video editing software like Apple Final Cut Pro. Slideshow does not synchronize the music to the lenght of the presentation, the added soundtrack will play from the point where it was inserted through the entire presentation or until another sound file is encountered. If the slideshow is shortest than the sound, it will fade out when the slideshow ends.
The software presentation is run loading in real-time the various slides and timing can differ from a computer to another: due to different speed and CPU power, the time taken to load and display images is not a constant. Even if the time shift from a situation to another should not differ so much, this is enough to prevent precise timing.
However, the movie export feature do create precisely timed presentations because rely on QuickTime which is one of the best high-end layer for multimedia.
Resuming:
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The total time displayed on status line of Slideshow should be considered as indicative.
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The duration of the software presentation and sync with soundtrack, either running on Slideshow or as a freestanding presentation for Mac or Windows, should be considered as indicative.
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The duration of the exported movie (any format) may be considered constant and you can expect any computer, either Mac or Windows, actually play the presentation with the same timing.
So, you can rely on the movie timing but not on the software presentation timing.