Default gallery width changes when activating plugin

This is the best gallery plugin I’ve found, but I have a small but annoying issue that when I activate the plugin, my gallery is suddenly off-center on my page. Digging through the CSS, I found that that culprit is:

#gallery-0 .gallery-item {
float: left;
margin-top: 10px;
text-align: center;
width: 24%;
}

When the plugin is disabled, the above code looks exactly the same, except the width is 25%, and the gallery centers properly on my page.

Edit: If I force 25% through custom css, the gallery aligns properly, but then I’m unable to choose less or more columns.

Any ideas what could be causing this?

http://ww.wp.xz.cn/plugins/simplest-gallery/