They're made of more and less efficient materials. As the TL rises, the costs of manufacture become more efficient. This process makes more of a given product available to consumer and improves the overall performance of said product.
Just look at home computers here on Earth, for an example. In just one Traveller TL the performance has increased better than 10-fold while the price has dropped by more than half. The PC on my desk right now cost me about $1800 give or take two years ago. In order to get anything like the same performance in 1977 [the year Traveller hit the shelves], that unit would take up MUCH more space, cost more to make, use more power, and break down more often. And, as the price I paid will tell you, I don't own anything like the kind of rig professional graphic designers or programmers own