How often it needs repairing, and how much it will cost you to put right. A car could break down five times, but only cost you £50 each time to fix, or it could break down once and set you back £1,000. That's why the Reliability Index figure combines several elements - cost, inconvenience and rate of failure - making it a 'real world' picture of car ownership.
Bad metrics, IMO. Inconvenience? When my car needs service I show up at 7am, sign a few papers, and am on my way in a complimentary loaner that is either a C class or GLK. When I had a Nissan Titan they had it all day for something minor, no loaner, and when I went to pick it up they told me the mechanic had been "working on the wrong truck all day" and mine would be staying overnight. The service writer made a big deal of dropping me off at a nearby rental company for a complimentary loaner, a tiny 3 cylinder car, and it took me 90 minutes of waiting in a shitty rental car lobby.
Just one example.