HTML added to editor when deleting shortcodes in Chrome browser

Example:

[shortcode1][/shortcode1]
[shortcode2][/shortcode2]

In the above example, using the google chrome web browser, while in visual mode in the post editor, highlight shortcode 1 by triple-clicking and click the delete key to remove it (remove the entire shortcode 1 element, open and close tags).

Now, click over to the “Text” view of the editor. The resulting html is:

<span style="line-height: 1.5em;">[shortcode2]</span>[/shortcode2]

I should also mention that I have an editor-style.css file which contains the following css rules:

@charset 'utf-8';
html .mceContentBody{max-width: 550px;}
body
{
    color: #555;
    font-family:'Calibri', Helvetica, Arial, sans-serif;
    text-rendering: optimizeLegibility;
    vertical-align: baseline;
    font-size:14px;
    line-height:1.5em;
}

I believe the editor is picking up the line-height:1.5em from the editor-style.css , but why?

Update: I’m using Google Chrome Version 26.0.1410.65