I'm sure you also noticed the other chord in this song that doesn't "fit" with the natural scale. That's the B7 in the bridge. In the bridge the song (rather jarringly) modulates to A from Am. Normally the 2 chord in the key of A would be Bm, but here we have B7 instead. The first time it's a secondary dominant (the V of V) (B7 to E7 to A) But the second time instead of B7 to E7, we get this lovely chromatic slide from B7 to Bb to Am to get us back into the key of Am and the verse.
I always loved that - it really tickles my brain at some level. It's my favorite part of the song.
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