$ bitbake-diffsigs -d -t refkit-image-update-test-a do_rootfs DEBUG: Signature file (previous): /fast/build/refkit/intel-corei7-64/sstate-cache/31/sstate:refkit-image-update-test-a:intel_corei7_64-refkit-linux:1.0:r0:intel_corei7_64:3:31cb4664537f2b75bd56fb3ab27b4372_ostree_prepare_rootfs.tgz.siginfo DEBUG: Signature file (latest): /fast/build/refkit/intel-corei7-64/sstate-cache/19/sstate:refkit-image-update-test-a:intel_corei7_64-refkit-linux:1.0:r0:intel_corei7_64:3:19d0014c03c3d8773ffaa1416f579616_ostree_publish_rootfs.tgz.siginfo ... Note how it picked up two siginfo files for two different tasks, both not being the do_rootfs that was meant to be compared. The actual diff then really is between two completely independent tasks. Probably the code which looks for siginfo files uses *<task name> as file pattern, without taking into account that one task may be the suffix of another.
Fix merged to master: http://git.yoctoproject.org/cgit/cgit.cgi/poky/commit/?id=d4fd7e2aa1ee1192e17d494faa52f4d52d041544