I discovered this upon installing image_caption_filter and my first test case had
border-width:2px;
which was matching in the search for the image width and resulted in my image-left and caption only being allowed 2px.

I fixed it by modifying image_caption_filter.module line 115
from

        if (preg_match('/width:\s*(\d+)px/i', $img_tag, $matches) == 1 |\
| preg_match ('/width=\"(\d+?)\"/i', $img_tag, $matches) == 1) {

to

        if (preg_match('/(\s+|^)width:\s*(\d+)px/i', $img_tag, $matches) == 1 |\
| preg_match ('/(\s+|^)width=\"(\d+?)\"/i', $img_tag, $matches) == 1) {

Thanks also to this issue that helped explain how to get the whole thing configured and styled.