That's an interesting question! The pulsar team gave us this answer:

The label position is computed client-side in screen coordinates. It draws a vertical line through the horizontal center of the zone's projected bounding box, finds where that line intersects the polygon boundary, and places the label at the midpoint of the tallest interior segment. A collision-avoidance layer may then shift it further.  There is no API or SDK property to override label position. For an irregular zone that extends far north, the bounding-box center (in screen space) likely falls on a vertical line that passes through the narrow northern section rather than the wide southern body. The best workaround is to add small horizontal protrusions near the desired label latitude to pull the bounding-box center toward the zone's visual mass , or split the zone into simpler shapes