In 163 amateur matches I had 8KOs, who am I? This guy would become one of the most devastating punchers in pro boxing, however in the am-game this guy was strictly a boxer.
Keep in mind though that for the vast majority of those bouts Tommy was just a kid and had not yet developed his man strength, and he was fighting at lower weights. He was not all that strong in the 130s, but developed more strength and power when he eventually went up to 147 as a pro. Plus, when he turned 16 or 17, he was fighting elite amateurs at 132 and 139, and it isn't easy to knock those guys out in 3 rounds with amateur rules. That's all I can think of for an explanation.