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 - 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.