How come deleted words re-appear after syncing?

That happens, when there are sync conflicts between the platforms. 

When changes have been made on several devices at the same time (or offline), these changes must be merged. In that case, deleted words are recreated because Wokabulary does not know if the word has been deleted on device A or created and device B. 

We don't want you to lose words, so Wokabulary tends to recreate words instead of deleting words. To prevent such merge conflicts, make sure to take all devices online before making any changes — so all devices will be on the same version.