The 6 to 6.5 hours you are getting is within the expected range when you factor in Conversation Awareness being active alongside ANC. Here is why:
Apple's advertised 8-hour battery life with ANC on is measured at 50% volume with only ANC active and no other features running. Conversation Awareness is not included in that baseline test because it is a computationally intensive feature that continuously analyzes the microphone input for speech detection. When Conversation Awareness is enabled, it runs the adaptive microphone array in a different mode and adds meaningful extra processing load compared to ANC alone. Enabling both ANC and Conversation Awareness together is closer to how Apple measures the 6-hour rating for Transparency mode plus features.
Also, the 8-hour figure is measured in a controlled lab environment with a specific audio content type (usually a standard frequency sweep) at exactly 50% volume. Real-world usage at higher volumes, with mixed content types like music, podcasts, and videos, and with Adaptive Audio switching between modes, consistently produces results 1 to 2 hours shorter than the spec.
So getting 6 to 6.5 hours with ANC plus Conversation Awareness at typical real-world volume is actually normal and not indicative of a problem with your specific unit.
If you want to maximize battery life: disable Conversation Awareness when you do not need it (press and hold the ANC button on the stem to toggle it), lower the volume slightly, and disable the Adaptive Audio mode in favor of plain ANC or Transparency. Those changes alone can push you toward 7 to 7.5 hours in practice.