Each slot is in fact a table header with specific colspan. When the gantt widget renders its headers a calculation is performed how much headers to be rendered depending on the time range of the gantt tasks. In your case there are six headers, the first and the last have colspan of 1, and the four in between have colspan of 4. If you inspect the dom elements, the first and the last header should have width of 30px since the total width is 120px divided by 4 since the maximum colspan is four and they have colspan of 1.
In case this is not the case I would like to ask for a dojo snippet with the exact implementation that you are using.
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