Sync doesn't have any user controls to designate where bookmarks should go, such as to or from a specific device. Bookmarks from all devices are merged on the Sync server and over time they become "balanced" so that each device has the appropriate bookmarks. "Mobile" bookmarks are handled slightly different than bookmarks for laptop / desktop devices; typically "mobile" bookmarks tend to be in different folders on desktop / laptop devices than bookmarks that are saved on those devices.

And the "merge" action doesn't happen all at once. The Sync server may take a few days to get everything arranged in what it considers the correct location on each device. The amount of time will vary quite a bit, depending on the amount of data involved for each user (the total number of bookmarks and other data), along with the load on the Sync server that each Firefox Account is "connected to"; along with the "time of day" load for each server.