Why is it that the design does not look like you expected? You gave
a picture of what you wanted. What you got back was exactly the things
you asked for but ... it just does not look the same.

Luke W has an interesting guess as why this is the case:

"What’s missing is what’s invisible: alignment and whitespace.

Often, user interface elements are not aligned. What’s
centered in the design doesn’t appear that way in what’s implemented.
What was vertically or horizontally aligned in the design appears
ragged.

Padding, or whitespace, often fares worse. In some
places, padding is gone; in others, there is too much. Padding is set
to different values, leading to columns and rows of varying widths.
Changes in padding and alignment can negatively impact readability and
obscure visual relationships that clarify how to use an interface.

Why is this the case? It’s not that the developers
deliberately modified the design. It’s not that they necessarily
consider alignment and whitespace to be unimportant. It’s just that
these elements in the user interface are often invisible to them.

Development teams are responsible for putting
interactive features and content into a product. Empty space is neither
feature nor content. Therefore, it is not a requirement. For a
designer, however, whitespace is often just as important as the content.

As a designer, I spend a lot of time adjusting whitespace to enable effective scanning of content. I also spend a lot of time refining alignment and padding to establish the right prioritization between user interface elements.
I utilize both of these design elements to guide users through the
interactions on a page. I use them to communicate what’s most
important, what’s related, and what needs attention. For designers,
these are key requirements of effective communication. And yes, there’s
a lot of evidence that shows what’s invisible does make a difference. "

 

Here is the article: http://www.uxmatters.com/MT/archives/000097.php