The Ubuntu kernel team maintain their own flavors of the Linux kernel for Ubuntu, because of the feature set Ubuntu depends on.
The Ubuntu kernel FAQ states:
What differentiates the Ubuntu Kernel from the upstream Linux Kernel?
Ubuntu kernels are rebased against stable releases only through the
development cycle, with many patches on top of the stable tag. Once
the final release is made the master branch is never rebased again.
While stable updates (post release) are usually applied, we sometimes
make patch decisions that are counter to the stable releases.
So, the best one could say is that Ubuntu kernels are only loosely
based on upstream stable. You'd have to examine the changelog to know
exactly what goes into a particular kernel.
With every Ubuntu kernel release, we attempt to remain as true to the
upstream Linux kernel as possible. However, there are inevitable
patches which we carry on top of the upstream Linux kernel which
differentiates the Ubuntu kernel from the upstream Linux kernel. This
document attempts to describe the general set of patches which are
carried and why: Kernel/FAQ/UbuntuDelta
Given an Ubuntu kernel package version how do we find the exact mainline release it is based on?
The exact upstream mainline tag from which the Ubuntu kernels were
forked can be found in the mainline kernel mapping table.
So the repository trees are not "synchronized". Rather, the Ubuntu team maintains the Ubuntu kernels with upstream Linux kernel patches, as well as other patches as needed.
Sidenote: Distributions that are derivatives of Ubuntu (which are actually off-topic here) usually use the Ubuntu kernels instead of maintaining their own kernel flavors.