min-width: 44.444em

On my desktop the content is limited to only 640px. Inspecting page, I realized that the responsible of such behavior is the following code in style.css

@media (min-width: 44.444em) {
		.site-branding,
		.social-container,
		.page-header,
		.entry-header,
		.post-gallery,
		.entry-content,
		.entry-summary,
		.page-content,
		.entry-footer,
		.post-navigation .nav-next hgroup,
		.post-navigation .nav-previous hgroup,
		.comments-container,
		.site-footer .blog-name,
		.site-info p {
			width: 640px;
		}
	}

I do not understand why, anyway. My screen is very large, so I have more than 500px on both sides of content. What is 44.444em? How large is?