Muddle over a repair

WITH my 2003 BA Fairmont that has still done only 70,000 km, I had problems with what I was first told was the wiring harness to my gearbox being too short. But when we took it in to get repaired I was told it was the computer, so I had to replace it. Though the car had done only 60,000km at the time, I was told it was six months out of warranty and I had to replace it at my own cost. Ford paid a percentage of the bill, but it still cost me more than $1000. Two weeks later I had the same problem and when I returned to Ford they found there was something else wrong. It looked like there really wasn't anything wrong with the computer after all. This time they fixed the car at no cost. I would like to know how these big companies get away with saying they have a three-year or 100,000km warranty, then not honour it.