Welcome to the RH community. I think you'll need to go down into the murky world of XHTML for us to help you solve this. If you highlight the text in the cell and click the HTML button at the top of the WYSIWYG, is there some tag that looks a little like this:
<p style="text-align: center;">
If so, this is what causing it. You could change it to <p style="text-align: left;"> from inside the HTML tab. Let us know how you get on.
The RoboColum(n) @robocolumn Colum McAndrew
Turns out the only HTML tag regarding centered text was for the whole table div, so changing that changed the alignment of the table as well. However, Peter's tip fixed the problem. Thank you both for your help!