Apache Theory Question

Hi,

I wanted to ask a question about The Shadows instrumental Apache. I noticed the song is in Am but in the verses the songs chords go:

Am to D major, then the next line it goes Dm - G. Now my question is, in Am there is no F# so the Dm chord (with a natural F note) would be the chord that fits correctly, yet there is a D major chord which has a F# note. I know this works fine with the song, but theory wise how do you explain this?

RockMan

Assuming the song key to.be Am, such a change (upon the Degree IV of the Key scale) often has the ability to enhance a new set of voices accounting for the, either inadvertently or not, voice-leading process implicit within the harmonic framework (this is not a remarkably elementary issue but anyway...): for instance, by such a change the relevant voice could be changed from, say, an inner voice (tenor, etc) onto the bass, etc. Greetings