Read it up and down, not left to right. A component's height decides the spare; its left-to-right position doesn't.
How often does it fail? A frequency, and on its own it tells you almost nothing about what a failure costs.
How long is it down when it does? The rare failure with a long-lead part can hurt more than the weekly five-minute fix.
What does the failure do to the rest of the system? Whole-line stop or one lane degraded. This drives the spares strategy.