That's because the mount/umount binaries on the Kindle do not have all the Whizz and Fiddles of the mount tool on Ubuntu. It's a downstripped version on the Kindle, for the sake of not spending precious space on the device. That's also true for other Unix tools on the device.
The umount on Ubuntu aims for comfort and - after not able to "really" umount b, it looks up the list of mounts and sees that b is mounted on a. It spares you the work of doing it "right" and does what you seem to want.
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