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The duration changes because of the irregular work week. If you look at your chart as you drag that 8h task across the week, you'll see the duration column continually changing. It is showing you how many days of the week will be covered by the bar for that task, and that number is clearly going to increase as the bar covers more days that resource isn't active.

When you change the value that says "this is how many hours are in a work day" all you are doing is changing the divisor in the calculation that converts small units into larger ones for purposes of display. As you noticed, the Gantt chart doesn't change (except if you are showing a textual duration figure next to a task). It doesn't have anything to do with the actual project work schedule, either the general one or the week-specific one or for any given resource's schedule. It's just a way to let you deal in more convenient units.