Not sure, but if those are SVN revisions they could be on different branches?

WebKit nightlies: http://nightly.webkit.org/

And they probably enabled or disabled some features, based on what they're working on.

For awhile (or, a few days, at least), Safari release was faster than WebKit nightlies on SunSpider, since they were working on strings in SFX (or something like that. Point I'm making is, release was faster and the slowdown was in the strings test).