Benchmarks are hard for comparing computing systems already. Design trade-offs are made all the time. As the nature of the software these systems run change over the time, so does the processor design changes to meet these changes. With more software taking advantage of the GPU there may be less effort in making your CPU handle floating points faster, so you can focus more on making integer math faster, or better threading...
2005 compared to 2015...
2005 - Desktop computing was King! Every user needed a Desktop/Laptop computer for basic computing needs.
2015 - Desktop is for business. Mobile system Smart Phones/Tablets are used for basic computing needs, the Desktop is reserved for more serious work.
2005 - Beginning of buzzword "Web 2.0" or the acceptance of JavaScript in browsers. Shortly before that most pages had nearly no JavaScript in they pages, if they were it was more for being a toy, at best data validation in a form. CSS features were also used in a very basic form. Browsers were still having problems with following the standards.
2015 - "Web 2.0" is so ingrained that we don't call it that anymore. Browsers have more or less settled down and started following the open standards, And JavaScript powers a good portion of the pages Display. the the N Tier type of environment it has became a top level User Interface Tier. Even with all the Slashdot upgrade hate. Most of us barely remember. clicking the Reply link, having to load a new page to enter in your text. And then the page would reload when you are done.
2005 - 32 bit was still the norm. Most software was in 32 bit, and you still needed compatibility for 16 bit apps.
2015 - 64 bit is finally here. There is legacy support for 32 bit, but finally 16bit is out.
These changes in how computing is used over the time, means processor design has to reweigh its tradeoffs it choose in previous systems, and move things around. Overall things are getting faster, but any one feature may not see an improvement or it may even regress.