The chord choices in Garageband are the chords found within the scale of C, plus Bb (even the Bdim - but, to be pedant, it should actually be a B7b5). Having Bb you can modulate (or "change the key") to F without having to switch scales etc.
Also, the fact that he found D, E and A among the results is probably because of modulations. It's VERY common for pop songs to modulate a whole step during some chorus near the end [1]. As mentioned, G, F and C are V, IV and I. If we modulate a whole step, from C to D, the V, IV and I are A, G and D. It would be nice to consider those modulations into the research.
About the key choice, I believe it's irrelevant. It depends a lot on what's your instrument (Bb, Eb is easier on brass instruments), your style (lots of Metal songs in the key of E because E is the lowest note on guitar), your tuning (lots of rock bands downtune their guitars to Eb or D etc), your proficiency, and, most important, the vocalists range.
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[1] Otis Reeding - My Girl, Celine Dion - Because You Loved Me (actually lots of songs by her), Monty Python/Eric Idle - Always Look On The Bride Side of Life, Talking Heads Nothing But Flowers (If you search "whole step key change" you'll get a bunch)
Also, the fact that he found D, E and A among the results is probably because of modulations. It's VERY common for pop songs to modulate a whole step during some chorus near the end [1]. As mentioned, G, F and C are V, IV and I. If we modulate a whole step, from C to D, the V, IV and I are A, G and D. It would be nice to consider those modulations into the research.
About the key choice, I believe it's irrelevant. It depends a lot on what's your instrument (Bb, Eb is easier on brass instruments), your style (lots of Metal songs in the key of E because E is the lowest note on guitar), your tuning (lots of rock bands downtune their guitars to Eb or D etc), your proficiency, and, most important, the vocalists range.
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[1] Otis Reeding - My Girl, Celine Dion - Because You Loved Me (actually lots of songs by her), Monty Python/Eric Idle - Always Look On The Bride Side of Life, Talking Heads Nothing But Flowers (If you search "whole step key change" you'll get a bunch)