To understand why, you have to understand what the default setting
of DynamicLineSpanner #'Y-extent does, namely to calculate the actual
extent of the included dynamics indications. Then, once this has done,
the placement of the full DynamicLineSpanner is determined using the
padding and staff-padding properties, that specify the distance from
the upper edge of the full DynamicLineSpanner to the closest note
and/or
staff. When you instead specify a fixed value of Y-extent, LilyPond
won't care about the actual extent of the dynamics and align it as if
the top was at, say, 1 (if you set #'Y-extent = #'(-1 . 1))
relative to
the internal reference point of the DynamicLineSpanner. To
complete the
picture, every element included in the DynamicLineSpanner, such as a
Hairpin or DynamicText makes its own alignment relative to the
reference point of the DynamicLineSpanner.
Excellent. Thank you so much for the explanation. And I think what I
was missing is the basic concept that turning Y-extent on is the point
at which we cause Lily to ignore the actual, graphic dimensions (in
the y direction) of various objects. Makes perfect sense.