For several years Mathematica has rendered PopupMenus in an inconsistent manner on Mac OS X. Through version 8, 9, and now 10, getting PopupMenus to render as per the current OS rendering seems to be very hit and miss. The default rendering for the past few years seems to be locked into the rendering from about OS X 10.6, even when running Mathematica on OS 10.9 or 10.10. For example, with the default stylesheet in V9 or V10:

However if we set a PopupMenuLabel style:

Cell[StyleData["PopupMenuLabel", StyleDefinitions -> StyleData["MenuLabel"]],
 FontSize->11,
 FontWeight->Plain,
 FontColor->RGBColor[0, 0, 0]]

All of a sudden the rendering switches to the current OS rendering (in this case 10.10)

I would have thought that the current OS rendering of popups should be deployed by default. However this does not solve the problem entirely because there are other occasions when you get a random mix of popup renderings!! (same stylesheet settings as per previous example)

and

This has been an ongoing problem for since at least version 8. It isn't clear what is causing the problem or why the default rendering is what Apple was using from 6 or 7 years ago rather than the current OS that is being run by the user.