What means "exceptional aspect ratio"?


Our Twitter Cards started getting cropped lately, for ex: https://twitter.com/nvineberg/status/311474595632922624 . Reading Twitter Card docs I found the following sentence:

“Images will not be cropped unless they have an exceptional aspect ratio.”

What does “exceptional aspect ratio” exactly mean and is that the cause of our problem? This example image outputs 960x1280 dimensions in twitter meta tags.


Actually I tried posting another Twitter Card with the same image dimension and for this one it scales Twitter Card correctly: https://twitter.com/MitiaBrevian/status/311569673999241216 . What can be causing the difference as the meta tags are the same?