Using MS Word Table Styles to Change Font Size

This is apparently a bug that Microsoft isn't going to fix.

I experienced the same problem and generalized the explanations in that question.

For that specific question, it comes from the fact that the Internal Root Style (IRS) is TNR, 10 pts. If the Paragraph style applied to the text in the table differs from that, it will override whatever you set in the Table style.

The solution is then to set the Normal Style to 10 pts, which will avoid him to override the IRS. Then you'll have to deal with it and create your own styles if you don't want to use 10 pts Font in your text body. My advise would be to create a "My Normal" style, and make every styles which previously inherits from the Normal style inherit from "My Normal" style. Note that the paragraph style applied to the table must also be 10 pts.