It may have been on a good spot in the starter motor when they tested it. That is why sometimes a starter will only click but you can hit it with a hammer or tap it with a prybar and the car will start. There are more than one set of contacts in that starter motor and if you land on a bad set you may be stuck. I do not trust those bench testers certain places use anyway. It took me 3 alternators to get a good one quite a long time ago and everyone bench tested good when I took it back. They asked me if I knew what I was doing and believe me, I had gone over everything because I couldn't believe it either. The 3rd one worked just fine. If the wires on your solenoid and batt terminals of your starter were clean and tight, it is probably bad.