Why are thumbnails first having about 10px or so cropped on all sides THEN resized to the thumbnail dimensions specified?

I've searched through the code but just can't seem to find where this extra is being removed from the edges

A pointer to the code where I can change this would be appreaciated

Thank

Thanks for reply.

I looked into imaging code a little more after posting this and saw mention of the need to crop images in order to maintain aspect ratio. The issue is that I've been comparing thumbs generated here with thumbs generated manually on an old site I'm wanting to convert to SS. The issue with many thumbs is that the cropping is of course not taking into account the content, so where an image (with larger height then width) had some text at the top, the auto-cropping is removing part of the bottom and top of the image and so cropping off the readable text. I guess no auto-resizing code can ever really factor in the content

When are thumbs generated? are they generated on the-fly or at import? Reason I ask is can I go and replace those in _resampled and be safe that they won't be overwritten?