What does PhoneGap support mean in Graded Browser Support chart?

What does PhoneGap support mean in Graded Browser Support chart?

In the graded browser support chart ( http://jquerymobile.com/gbs/) PhoneGap is similar, but not quite as good, as the Native column. My initial assumption is that PhoneGap uses the default browser for each device, which would mean it doesn't require its own column in this chart. Is this not true? I noticed John Resig mentions PhoneGap "uses WebKit as its rendering engine" in this presentation ( http://www.slideshare.net/jeresig/testing-mobile-javascript?from=ss_embed), which would imply the PhoneGap folks developed their own WebKit browser that works across popular mobile devices.