The Taboo was complex and not fully understood:
* Selective Activation: It's possible the Taboo wasn't triggered by simply saying "Voldemort" but by specific intentions or contexts. The Death Eaters might have been listening for whispers of an attack, plans for rebellion, or any mention of their master in a way that posed a threat.
* Specific Circumstances: The Taboo likely wouldn't work in casual conversation. The trio saying "Voldemort" while at Grimmauld Place, especially when planning actions against him, could have triggered the Death Eaters' awareness.
* Intention and Emotion: The way the trio said "Voldemort" in private could have been significantly different than how the Death Eaters used it. Their anger, fear, or sense of urgency might have influenced the spell's reaction.
The Taboo wasn't always perfect:
* Mistakes: The Taboo wasn't foolproof. We see instances where it doesn't work as expected. The Death Eaters were alerted to Harry's whereabouts when he unknowingly said "Voldemort" in the Ministry of Magic, but they didn't react to his use of the name earlier in the book.
* Limitations: The Taboo might have had limitations or loopholes. Perhaps the Death Eaters had a limited number of spells at their disposal or their focus was elsewhere during certain moments.
The narrative and plot convenience:
* Dramatic Effect: J.K. Rowling used the Taboo to create tension and drive the plot. The Death Eaters' ability to locate Harry through the Taboo heightened the danger and added urgency to his mission.
* Avoiding the Obvious: If the Taboo worked flawlessly, it would have made Harry's mission impossible. Having a loophole allowed for more complexity and suspense.
Conclusion:
The exact mechanics of the Taboo are never fully explained, leaving room for interpretation. It's plausible that the trio's casual mention of Voldemort didn't trigger the spell because it lacked the specific intention or context the Death Eaters were listening for. The Taboo's limitations and Rowling's narrative choices also play a role in explaining why it didn't always work as expected.
Ultimately, it's a question that each reader can ponder and come to their own conclusions about. It's a testament to the rich and complex world Rowling created, where magic, plot, and character motivations intertwine in fascinating ways.