Einar,
I tried your code out, and also created a test form where I defined a tablestyle using the designer, and ran into the same issue. No matter how I resized the columns, I got 75 for a width when I checked the gridcolumnstyle, just like you.
I've never run into this before, because I've always defined a table style and column definitions through code, not through the designer, and it's always worked fine. Sorry that I don't have an answer. I'll look at this again later today, unless someone else has some ideas.
Kevin