Quotes and other scopes like Cdata seems to have higher importance over all the others, you get the same result if you try something like:
$foo = '<div class="baz">bar</bar>';
the html tags wont be highlighted as such, 'coz actually it's a quoted text string.
To me the highlight is working "as expected", i'll be surprised to see a quoted string with some part highlighted in different colors, doing so will lower the readability a lot imho...
hope it helps