Comment 2 for bug 1247700

At first, it looks like this is simply because "/boot/vmlinuz-3.12.0-031200rc7-generic" is lexicographically greater than "/boot/vmlinuz-3.12.0-031200-generic", but I haven't actually looked at the source for grub2, and something in /etc/grub.d/10_linux may also be responsible for sorting the entries.

I haven't come across documentation saying whether or not this is expected behavior, but in bug 476540 it was mentioned how the entry sorting was changed to the algorithm used by dpkg. The discussion in http://savannah.gnu.org/bugs/?42597 would indicate this is issue is due to the intended behavior of /etc/grub.d/10_linux, at least upstream. There are several other similar discussions grub2's kernel menu sorting and /etc/grub.d/10_linux.

There was a similar issue reported with update-initramfs that was resolved by using the linux-version command (from linux-base): cf. https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=770394 . This might be something to incorporate into /etc/grub.d/10_linux as a solution to this and similar issues.

I happened to be testing mainline kernels at the moment, and as a workaround I use grub-customizer to specify which kernel to boot.