It's at specific widths. I've seen other responsive sites behave like this and I presume it's just something with the media queries in the style sheet. If you continue to narrow your browser you'll see that it does respond correctly.
One is clearly an example of responsive on a phone or tablet, but the other appears to be an example of broken responsive where the content got truncated as the OP of this thread mentions?
Why show an example of broken responsive design?