Ah, this I can explain. In the chorus the single-note riff is replaced with root-fifth chords, or 5 chords; e.g, D becomes D5, some combination of the notes D and A. Varying from the original, I put the 5th of the chord as the lowest note; e.g., for D I used A-D-A. You’re probably hearing that low A.
I'm glad I read some of this again because last night I played from a different video for Seven Nation Army White Stripes. I thought I heard a half step between the last two notes in your "riff" (new term for me). I'm sure I heard a half step.
Evidently you knew that and changed it to the full step which works better.
Maybe I will get my keyboard checked and try some of the tips on yt for playing by ear. A familiar melody I can pick out the right hand pretty accurately (not a really tricky one like um Dances with Wolves). I just didn't know what chords to put with it on the bottom.
And I learned all my chords years ago, including the more advanced ones, but can't play inversions and variations quickly. So I am working on the circle of chords and progressions and will see if I can pick up some more. Even if i am old, maybe I can still learn a little more.
Yeah maybe I can pick out most of Dances with Wolves. I have the music with guitar chords for that and other favs so can fall back on them. I just never liked the piano arrangements on pop music or even traditional music, too blocky and blah. But my ability to play well using the hymn books, etc., I do miss way to many notes, sight reading it's called. I always had to practice.
Way more than you wanted to know lol.