That's because they hadn't got round to casting voice actors yet and wanted to release something to show, so they hired different people at the time. This thing happens quite a lot.
Yeah, it's like people have never even heard of the concept of a "placeholder". Voice acting is probably one of the last things to be done for the game. Why? Because you want to make sure you don't need to change the script before you bring in the voice actors, because you don't want to waste money paying them to record lines, then have to pay for another session to record different lines.
So, until late in the game's dev cycle, you can't really say that dialogue won't change (maybe the writers decide to change some Lore, and need to update the script, or maybe the dungeon/quest designer decides they need to add/change some lines, etc).
But, you want to get a trailer out in time for some tradeshow or something, so you have one of your 'staff voice actors' who works with you all the time do a voice for a bit.