"I just assumed that any excess inventory would be sold at TKTS. It this not always true? Does this mean that the producers of Legally Blonde would rather have empty seats than selling the remaining seats at TKTS?"
The producers of each show make the decision whether to release tickets to TKTS and how much of a discount to provide. Keeping tickets off TKTS, even if some are available, helps fuel the perception that a show is "sold out" and a "hot ticket." It also forces people to purchase tickets at full price. Producers who thus choose to keep their tickets off TKTS even though there are empty seats have made the economic decision that the perception of being sold out, plus the higher ticket prices, outweighs any financial gain in selling the additional seats at half price.
After all, the vast majority of the theatre-going public does NOT read the weekly grosses, and so really have no idea whether or not a show is actually selling out.
"This is wanting something, this is reaching for it,
This is wishing that a moment would arrive.
This is taking chances, this is almost touching, what the beauty is." --The Light in the Piazza