Your question is: "Does anyone know why this might be the case?" The answer is that the ascender property of the Japanese font is smaller than the Classic Robot one. This can be fixed by generating a custom Japanese font that has an ascender property large enough to make it match the spacing you get from the other font. This can be done by downloading the Apple Font Tool Suite and following the instructions posted in this answer.
Also for buttons you can solve the issue by increasing the insets:
myButton.contentEdgeInsets = UIEdgeInsetsMake(15.0, 0.0, 0.0, 0.0);
You can specify a different value for each of the four insets (top, left, bottom, right). A positive value shrinks, or insets, that edge—moving it closer to the center of the button. A negative value expands, or outsets, that edge. This works for the button image and button title.