This is explained in the release notes for Precise:
By default, the 12.04.2 point release will ship with a newer
3.5.0-23.35 Ubuntu kernel from Quantal, and a matching X.org stack. This is based on the 18.104.22.168 Extended Upstream Stable Kernel
Release...The purpose of providing a newer kernel in the 12.04.2 point
release is for hardware enablement.
This only applies to Ubuntu, Edubuntu, and Mythbuntu.
In short, you get the newer kernel if you install from
12.04.02; but keep the older kernel if you update
12.04.01, unless you take steps to enable the update.
For more information on how to do that and various policies and procedures on this issue, see LTS Enablement Stacks.
From the above link:
Anyone wishing to opt into the hardware enablement stack for Precise
may do so by installing the linux-generic-lts- and
For example to install the Quantal hardware enablement packages in
Precise, please run the following command:
sudo apt-get install --install-recommends linux-generic-lts-quantal xserver-xorg-lts-quantal
To install the Raring hardware enablement packages in Precise, please
run instead the following command:
sudo apt-get install --install-recommends linux-generic-lts-raring xserver-xorg-lts-raring libgl1-mesa-glx-lts-raring
See the above link for additional specifics regarding the exact policies and procedures regarding the support, maintenance, and upgrade paths for these hardware enablement stacks.
The support schedule for the kernels is given at the Version Matrix