What does font size really mean in ios?

Seems font size doesn't determine anything.

Here is two line, one use font zapfino, another use system font. They both has size 30, but the line height or other font metrics is really different.

This pic is draw with YYLabel which use coretext layout and draw the text, use this framework we can easily see the border of one glyph.

Below is another pic which use two UILabel with different font and preview it in StroyBoard.

So what does font size mean in coretext when layout one line?