It isn't clear to me that is it just comparing the javascript engines like for like. For all we know Google throws more operating system resources at processing

It isn't clear to me that is it just comparing the javascript engines like for like. For all we know Google throws more operating system resources at processing the a single page than IE does natively. It may also be a battle of architecture (where each chrome page is a separate process with its own engine). So it may not just be the skill of the engine developer, but competing architectures.