MS forked Trident with different .dlls.
Problem is the rendering engine goes through a crap ton of if/else statements for specific workarounds and compatibility even without the legacy modes. It is crusty. Remove it for better performance and websites which detect IE and feed ancient code break and corporations freaks out.
So Spartan is like Firefox. It removes crap from Netscape/Mozilla. So if a site is in trusted zone like an intranet page it loads the older trident .dlls for compatibility. Other than that the newer versions focuses on standards on a much cleaner and fresher slate.
IE is a terrible brand. It was an awesome brand in the 1990s but MS really screwed up last decade with IE 6 and letting it rot for many many many years. Now the standards are so radically different it needs modes and workarounds to keep intranet apps and modern websites happy. Meanwhile Chrome doesn't have this problem but I have notice it slow down and take more ram recently.
Ms wants this to be like its firefox/chrome. A new slate to build upon which is fast and lean. Also rumor has it MS will use Chrome's pepper APIs for extensions and will have add-ons. Another thing IE has been missing for a decade now.