It seems that the plugin is reading the text contents as well. Consider the code below:
HTML:
<h1 class="text">Hello</h1>
Compiled CSS:
.text {
color: red;
}
.hello {
background: blue;
}
The .hello class is supposed to be removed from the compiled CSS since such class name doesn't exist in the HTML file, but it's still there because of h1's text content, which is "Hello".