image_style_url() function not giving proper image in drupal 7
Then it is likely the rule you are referencing is not configured to do what you want it to do. It may be cropping instead of scaling or do a scale and crop.

Edit the image style at: admin/config/media/image-styles/edit

Check to see if it is scaling or cropping. Adjust it or create a new one and reference that in your code.