Leading is the space measured from the baseline to the bottom of the bounding box of the glyphs above.
The possible missing factor here is, when there are multiple lines of type, the visual representation of leading includes both space above the baseline and the bottom of the glyph bounding box for the glyphs themselves. In other words, visually, the full leading size is related to both the space above the line and the bounding box of the glyphs.
In your images there is no line of text above the first line, so really there's only leading applied there, and no bounding area related to the bottom of glyphs. Without a line of type above it, there is no "bottom space" to see, resulting in a visually smaller space.
Really the top line shows only the leading. Add the area for the bounding box which falls below the baseline and you get the same space.
This can be seen merely by checking the difference....
When you overlap the area at the bottom of the text, to show the bottom of the glyph bounding box, the leading visually appears the same as it is between lines.
This is also why, with a single line of type, altering leading has no apparent effect. There is no glyph above to create space between.
Before I fully understood the question.....
As for the general top space as I understand it....
Space (top or bottom) of glyphs is generally provided for other glyphs which contain diacritics or swashes. This is done so that all the base glyph shapes line up.
The actual bounding area for the glyphs can be set to either encompass the entire glyph with "extra" content. Or to sort of "hang it" over the bounding area. Kind of depends on construction of the font itself. The more it "hangs over" the more glyphs may overlap with standard leading values.
A font which does not contain diacritics or other OpenType features may not have this additional space.