It's because you installed kernel-power before, so of course there's gonna be the kernel-power-bootimg and not the kernel-cssu-bootimg. CSSU is installing kernel-cssu-bootimg only if kernel-power isn't already installed as far as I understood it.

That's why I said there might be something wrong going on with your combination of cssu-thumb+u-boot+backupmenu+kernel-power. It shouldn't but it's a possibility. Maybe you could just try to reflash and ommit installing kernel-power this time so that CSSU installs kernel-cssu instead.