This aspect of voiceover recording is perfectly normal. You can record the same person in the same room using the same microphone and same recording levels on two different days and their voice will sound different.
I do all my own voiceover using identical location and setups and still find this to be the case, even though I am a very experienced public speaker. I have learned to minimise the differences over the years, but I've never found a way to eliminate them entirely.
