We are going to get a little technical here. But I will try to answer your question.
Many guitarists (including me) play chords by using “shapes” or moveable chords they’ve memorized, and there is nothing wrong with that. But I like to analyze harmonies or melodies for that matter, from a pianist point of view.
So the following explanation is a piano oriented subject, that answers your question and can be applied to guitar.

Pianists sometimes play ROOTED VOICINGS, also known as Bud Powell voicings. These type of chords place the root deep-seated on the bass to provide a solid and open sound.
Rooted chords can be voiced as: 1-5, 1-3-7, 1-7 and 1-3.

Bellow we have two barre chords: A and A7.
Notice how the notes in red form an A power chord, simulating a rooted voicing type 1-5.


e--------5-----------------5--------------------------------
B--------5-----------------5-----------------------------------
G--------6-----------------6------------------------------------
D--------7-----------------5-----------------------------------
A--------7-----------------7----------------------------------
E--------5-----------------5--------------------------------------

Now let’s focus on the rest of the notes.
In the case of the first chord, the notes in black correspond to an A major chord; where the tonic has been doubled 1-3-5-8.

The second chord is a Four-note chord.
Piano players (especially in jazz) increase the spacing by positioning one or more notes one octave bellow. This technique is known as DROPPED VOICINGS.
So let’s take the A7 chord: (A-C#-E-G)
And let’s put it in first inversion: (C#-E-G-A)
Now lets drop the G note one octave bellow: (G-C#-E-A)

As you can see the second chord corresponds to a Dominant chord in first inversion; where the flatted seventh has been dropped one octave bellow.

Now I know that some readers are going to say…”Come on, it is just another voicing of the Dominant Chord’’ … and I agree because that is just what it is. But now you know the theory behind it and why the chord is voiced in that particular way.


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