The way I experience it, noise cancelling works best on low frequencies of whatever you are listening. It complements passive noise isolation, which work best for high frequencies. Voice is usually in the middle, and it tends to be less attenuated, often that's a feature: you don't want do hear background noise, but you want to hear when people talk to you.

Noise cancelling headphones are never just about cancellation, they are always a combination of both cancellation and isolation working together, and it involves a lot of tradeoffs.