i don't doubt that it worked for you but that is not the norm. google confirms that "protected" i.e. paid for apps cannot be backed up in non-rooted android 4.1 or later
e.g. this
http://nelenkov.blogspot.ru/2012/07/...elly-bean.html
'From Jelly Bean and forward, paid apps in Google Play are encrypted with a device-specific key before they are delivered and stored on the device.'.
i suspect your nexus is running an older-than-jelly-bean version of android and that's why you were able to do it, and that your galaxyTab is not which which explains that outcome also.
i sideloaded the bookerly font into mantano but did not rate it and reverted to caecilia which I had previously sideloaded , so I have the same font set as default for all my readers/reader apps. I did note that I got exactly the same lines per screen/words per line counts with bookerly as with caecilia.