Time Machine backing up way more space than hard drive contains
Not exactly sure since I haven’t used TM with Mojave yet however, I may help point you in the right direction or stimulate a more accurate answer.  TM can backup all files including temp files, deleted files, cache files and even USB drives attached. If you were making a lot of changes during the initial backup, the initial file size will compensate for that. Also Mojave utilizes APFS which contains nested volumes and containers. There are additional formatting files outside of the readable HD within the volumes and containers that could be getting backed up to make sure you will be able to install from backup correctly.  It could also be backing up your recovery HD. If you run diskUtil list in terminal you can see the size of any hidden volumes and containers.  

I wonder since APFS uses the new file reference system to save space while keeping track of duplicated and edited files, maybe that could be play a role as well. TM HD might be saving both or be adding additional reference files for the locals snapshots? In the long run, TM will be writing over the older saved backup files with newer content. So I don’t think you should pry too much to get that additional space back. 

I have had issues in the past trying to restore from a TM HD that was the same size as my primary HD and wouldn’t recommend it. Using a drive at least 2/3 times larger is the right way to go.