[mcp6453[_2_]]

Maybe the musically inclined here can help me with this question. As background, I'm a terribly amateur musician, music
theory is a mystery to me, I absolutely can't sing, and the only way I can properly tune a guitar is with a tuner.
Yesterday for no reason, while playing a chord on my guitar that sounds a lot like "the chord" in the Beatles' "A Hard
Day's Night", I moved it up to the seventh or ninth fret. (I don't remember which.) As soon as I struck the chord, I
immediately recognized it as the same as or similar to the opening chord in "Venus", but Shocking Blue. I then loaded
"Venus" on my computer and hit play. Not only does the chord sound right to me, but it was in the exact key.

If I played it in a different key, it doesn't sound like the song. How does that happen?