New kernel changes default to 0 in menu.lst

In Karmic, reboot after new kernel install (caused by Update Manager) causes 'Must be authenticated' by grub when it tries to boot using old kernel. This is because kernel update results in Default value in menu.lst being changed from 0 to 2. I amend menu.lst and it boots OK using new kernel. I changed menu.lst (before upgrade from Jaunty to Karmic) so that it had an md5 password set and changed Howmany valuer to 3. Why is this change to Default value happening on kernel change and I how do I prevent it?
PS on another PC using Jaunty, this change to Default value does not occur even though it also has password md5 set and Howmany set to 3.