By default, FlatPress replaces some characters (or character combinations) within your entries and static pages:

• Two hyphens (“--”) become an en dash (“–”), three (“---”) an em dash (“—”)
• Three dots (“...”) become an ellipsis (“…”)
• Quotations marks become typographic: "text" will become “text”
• Trademark sign: “Text (tm)” is being replaced with “Text ™”
• Multiplication sign: “2x3” is displayed as “2×3”

… and a few more.

If you would like FlatPress not to do these character replacements, just tell it to :)

After the first two lines of your index.php, just add the remove_filter() call as shown:

<?php
include 'defaults.php';
include INCLUDES_DIR . 'includes.php';

// ADD THIS LINE:
remove_filter('the_content', 'wptexturize');

define('MOD_INDEX', 1);

...

The described replacements take place in the function wptexturize() in fp-includes/core/core.wp-formatting.php. Please refer there for a detailled view into the replacement logic.