Zen grids result in too wide sidebars & content when using sass

Hi,

I installed compass (gem install compass) and zen 7.x-5.5 (drush dl zen; drush zen mytheme --without-rtl).
Using mytheme works fine with a drupal install. Pages with two sidebars+content are fine.
However, if I use compass to compile the sass files that come with zen 7.x-5.5 without changing anything in them, I can generate a new styles.css. If I use this file with the same drupal install, all pages become way too wide for the screen. Here is the relevant section in styles.css:

/**
* Use 5 grid columns for larger screens.
*/
@media all and (min-width: 960px) {
....
/**
* The layout when there are two sidebars.
*/
/* line 194, ../sass/layouts/_responsive.scss */
.two-sidebars {
/* Span 3 columns, starting in 2nd column from left. */
/* Span 1 column, starting in 1st column from left. */
/* Span 1 column, starting in 5th column from left. */
}
/* line 196, ../sass/layouts/_responsive.scss */
.two-sidebars #content {
float: left;
width: 300%;
margin-left: 100%;
margin-right: -400%;
}
/* line 201, ../sass/layouts/_responsive.scss */
.two-sidebars .region-sidebar-first {
float: left;
width: 100%;
margin-left: 0%;
margin-right: -100%;
}
/* line 206, ../sass/layouts/_responsive.scss */
.two-sidebars .region-sidebar-second {
float: left;
width: 100%;
margin-left: 400%;
margin-right: -500%;
}
}

That's obviously wrong. The percentages in the responsive.css file that comes with the source are all <=100%. Those are the right numbers.
I am beginner as a zen, scss user. What am I doing wrong? How can I get right percentage numbers with sass/scss?

Yours: Laszlo