I feel the same way about other NPCs, such as the Undaunted pledge-givers. You can tell the lines are being read by the same actors, but sometimes there's a huge difference between their common greetings and their lines about today's dungeon. You can tell they were recorded at different times, and possibly under different circumstances, because the audio quality/clarity, the volume, and the timbre of the voice is just so different. I've sometimes wondered whether there's any kind of after-the-fact audio processing that ZOS could do to make the audio files sound more consistent.
If the same studio is used for voice recording, they'll keep notes of any particulars of the setup, including processing chain settings, so that the next time a session for that project/voice actor comes around, they'll be able to start up more or less where they left off and have consistent results.
But if things change -- different studio, new equipment, whatever -- the sound is very likely to be different. Sometimes subtly, sometimes more noticeably. Even with modern processing software, there can still be differences.