The first set of checkboxes has the CSS class radio-checkbox-group applied to its containing element.
In addition to behaving like a standard checkbox (toggling state when you click either the checkbox or its label)
these checkboxes can also behave like radio buttons.
If you long-press (or Option-click on MacOS or Alt-click on Windows) then that checkbox is selected and all of the others are deselected.
Long-pressing again selects all checkboxes.
If you have a set of checkboxes that are often, but not always, mutually exclusive this is a good user-experience enhancement. You can select peach rhubarb pie with a single long-press on peach followed by a click on rhubarb, whereas selecting both rational and imaginary numbers — but no others — requires four separate clicks on the items that you are not interested in.