Normally, when hovering the mouse pointer over a list of selectable items, the item under the mouse pointer becomes "hot":
This is an important UF (User Feedback) feature in standard UI (User Interface). It is described in many UIGLs (User Interface Guidelines).
It would be nice and helpful if this would also happen in a KM Macro Palette:
This is common on the web (probably in part because of the incredibly inconsistent UI used on web pages).
It is not common on Mac controls. Hovering occasionally will bring up a tooltip (which indeed it does in palettes without text in Keyboard Maestro).
There are a few places it happens in the Mac UI (eg the Safari preferences toolbar), but most do not highlight until you click down and then highlight as you move off or on the control to indicate that the action can be canceled (eg the Safari preference panes, as well the System Preferences main panel).