For the most part, I'm betting it's just the default way the code is generated by the content hosting sites different people are using. When you click the "share" link in Flickr, for example, it automatically generates the code to both show the photo and embed a link back to Flickr. It also puts the title and Flickr user name underneath. On Photobucket, on the other hand, the default BB code that's generated doesn't have anything except the IMG code to show the photo. Multiple other sites are also being used, and they all vary slightly.
And, of course, the person posting can choose to edit how the link appears in their post before they submit it by editing the code.
For my photos, I don't use Flickr, so I post them either from my Smugmug account (for now, the non-Disney ones) or directly from my own web space (I have several sites that I work on from time to time, mostly to keep my programming skills up, I dump all of my "finished" photos into the file structure there so I can link directly to them)
With that said, this is one of them from said file structure. It was formerly a throw-away until I reworked it some. It's far from perfect, but I like it considerably more than I did before.
Canon T1i - 50mm - 1/60s @ f/2.2 - ISO 250
Converted to Black & White via Lightroom 3