Interesting post about the multitude of varieties and flavours of WebKit that are available that is worth a read.
What is important to understand is the WebKit is an engine, it is not a browser. Products take a snapshot of WebKit and then incorporate it into their own products with a specific set of features enabled (HTML5, SVG, Web Workers, Inspector, SQL db, ...) and their own browser decorator or presentation functionality (ie Safari, Chrome, Iris, ...) snapshot it, ship it and maintain it for as long as they need.
Here at Crank we have ported and optimized the WebKit engine over half a dozen times in order to accomadate different operating systems, CPU architectures and rendering technologies. No two of our ports have been the same because our clients, mostly building embedded products, have very specific needs that we cater our WebKit customization efforts towards. The needs of an in-car navigation system are different from those of a kitchen appliance which are different from those of a stereo receiver, digital printer and camera.
WebKit offers a great technology baseline that can be readily customized and ported. Some of those changes make it back into the baseline, others do not. Given our experience with WebKit I'm not surprised by the diversity of experience.