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I think I'm just lazy. Similarly, what's is there a reason why the hand position is switched around between A, Am and A7? My index stays on the B string for all of those.
The positions that are taught sometimes tend to be for reasons of what chords you are likely to be moving to/from, and having "anchor" fingers that help you not have to lift all your fingers off the strings between chords. For example, the Justin Sandercoe method of fretting an Amaj is to have your index finger on the G string and ring finger on the B string, so that you can leave the index finger where it is when you switch to/from Dmaj, and just slide the ring finger up/down one fret. So you only need to move your middle finger from the E string over to the D string.
That chord chart reminds me of another weird thing I still do. My Dmaj shape is just a Dmin with index on the 2nd fret.
I think I'm just lazy. Similarly, what's is there a reason why the hand position is switched around between A, Am and A7? My index stays on the B string for all of those.