they think:
1) front center and rear center: very slightly variable, a balanced weight distribution that classically works and scales a small amount for rider size
2) head tube angle: highly variable, to adjust for your the reach
3) rake: highly variable, to correct the trail
4) top tube length: whatever connects the dots

you think:
1) rake and head tube angle and trail and rear center: almost locked, 43ish and 73ish and 57ish and 400ish, or whatever
2) top tube length: highly variable, to adjust for a persons reach
3) front center: whatever ends up happening

youll end up with the same contact points, head tube length, and trail but different front centers, head tube angle, rake, and top tube length.