Chord variations above a 7th have always confused me. Penny was kind enough to send me a video of her playing Welcome To West Texas so I could see her left hand. As I was working through it, I saw some differences between how she played it and how I was playing it (because… you know… she’s Penny Clark and she’s incredibly talented and I’m me.)
She was playing a G6 where I was playing a G13. Now, the ONLY reason I know it was a G13 is because I was using an online guitar chord finder app ( https://jguitar.com/chordname ) not because I knew it was a G13.
But when I saw her G6, it hit me that 6+7=13. So I started looking at other variations. Turns out G9 is the 7th plus an added 2. G11 is the 7th plus an added 4. Math and music? Get out of here! 
So I started reading online. It turns out, depending on who wrote what I was reading, that TECHNICALLY a real 13th must also include the 7th, 9th, and 11th notes to be considered a true 13th. I don’t even think that’s possible to play on the guitar… but I know that the G13 I play sounds nearly identical to the G6 that Penny played, especially when I’m not barre-ing hard enough and that 7th note is muted. Now I know why! I’m very excited about this. 
So, in summary, I’m feeling pretty dumb for just now realizing this, but I hope it helps someone.