Why does Chrome read the svg circle radius from the style attribute?
The current published version of the SVG specification is 1.1 it states that the radius of a circle is an attribute and not a CSS property. That's what IE implements.
There is a new version of the SVG specification being worked on, version 2. In it, the radius of a circle (along with many other things that are currently attributes) would be CSS properties. That's what Chrome implements.
Chrome is experimenting with implementing parts of the SVG 2 specification as is Firefox. Different UAs have implemented different parts of the SVG 2 specification as they investigate SVG 2 prior to its completion to prove that it's feasible to implement it.