There is a difference between changes in version of Ubuntu to changes within the version. You are comparing a distribution upgrade with a normal upgrade of packages.
Within a release there is some consistency. The underlying structure will always be Karmic. The kernel version may change from 2.6.32.x to 2.6.31.y, but not likely change to to say 2.6.32, unless you choose to upgrade the kernel yourself. The reason is that some people want minimal change. Those who want more change usually opt for a distribution upgrade.
BTW, do you realise that Karmic support is only for six more months? If you want to upgrade then it would be a good time to upgrade to Lucid which is LTS and has 2 1/2 more years of support, plus you get a newer kernel. :)