An arrow, for example, with a placement `bottom`, can still be placed in multiple locations, including `left: 100%` and probably `left: 0%`, depending on either a preset configuration or automatically calculated based on the positioning of the popover/tooltip on the page.

If you look at the Clear designs, you'll see that the arrows are not centered in the middle of the popover, like Bootstrap does, but rather on one side or another.  Currently, all the popovers are using an arrow positioned at `left: 100%`, but there will have to be support for additional positions in the near future.